Earth Notes: Saving Electricity and Gas

How we cut carbon emissions at home from 6 tonnes of CO2 per year to zero, for all 4 of us!
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If you cut waste and use energy more effectively, you'll save money on your bills, be more comfortable, and save the planet to boot!

The basics are simple:

  • Turn gadgets off when you're not using them, eg TVs, games consoles, lights.
  • Set TVs, computers and so on to turn themselves off or sleep automatically when you haven't used them for a while; costs no effort or money but makes a difference!
  • Don't leave lighting and heating on in rooms that you're not using; ~50% of the money spent on heating is wasted that way, costing hundreds of £s each year.
  • When buying new appliances and lights etc, buy the most energy efficient that you can find and afford.
  • Don't keep running old freezers in your garage if they are more than a decade old, or extra 'beer' fridges; they are likely huge money and electricity guzzlers!

Look at the 'See also' links for other great ideas to save your wallet.


Every unit (kWh, kilowatt-hour) of electricity saved avoids emission of CO2 (up to 2015 assuming a little over 400g CO2 emissions per kWh used has been a reasonable estimate).

Every kWh of gas (burnt on-site for heat) saved avoids production of ~0.19kg of CO2.

If you want to see some of the details (and data) for how we kept our comfort but reduced our energy consumption by about a factor of three and went carbon negative doing so, please read on.

We started our trek back in the mists (well, June) of 2007...

June 2007

PM 230 plug-in power meter

To that end I bought a PM230 kWh meter meter to accurately measure what energy/power a device is consuming, even when apparently off (eg the dreaded "standby" or "vampire" consumption).

I measured the power used by some gadgets around the house and in my study, to see what is worth turning off or even replacing, given that some is old.

Having done this survey (June 2007), we're going to make more effort to turn things such as the PC and cable TV box and stereo off completely, but the biggest potential saving is to replace my aging servers with one low-power laptop (still providing essentially the same services) for less than 5% of the electricity. At some point the laptop might even be moved 'off-grid' to solar power, but that's a separate consideration. The laptop should pay for itself in one year in reduced electricity bills. Thanks very much to Adam of [defunct: ""] XePhi for suggesting and devising configurations and power-saving tweaks, measuring power draw of his stock kit, etc!

I'm also replacing the last few old-style incandescent (filament) bulbs with modern CFLs for about one 5th the power consumption. In particular, the main living-room dimmer switch (which no one liked anyway) has been replaced and the 3x60W bulbs are being swapped out for CFLs. Only the bathroom light and one exterior bulb won't be replaced with CFLs since I'm concerned about condensation getting into the electronics and damaging them or even risking a fire. (Update Aug 2007: testing CFL in bathroom and watching carefully for problems; no problems observed as of Jun 2011.) (Update Jun 2008: our gas supplier, Altantic Electric and Gas, sent us two more CFLs unbidden, which creates an interesting problem since we already have CFLs virtually everywhere possible. Maybe I'll see if I can give them away creatively to neighbours without.)

Without giving up any creature comforts we should be able to reduce electricity use at home near 5x, to maybe ~3kWh/day base load with maybe another 3kWh/day for one wash and one dry with the washing machine and one dishwasher load and other sundries each day, ie ~200kWh/month (£20/month); probably somewhat higher in the winter when more time is spent indoors, etc. I confirmed with our power company that our current load (as of June 2007) is ~30kWh/day. (Update Jun 2011: we're down to ~4kWh/day.)

July 2007

As of late July 2007 the old server farm has been turned off and replaced with a much lower-power solution saving ~640W+ every hour of every day.

graph of electricity usage to August 2007 graph of electricity usage to October 2007

(Update Oct 2009: the servers had by now been migrated to an 'embedded' Linux system consuming ~4W (excluding the ADSL modem/router/WiFi), completely silent, very small including its no-moving-parts bulk storage, and running entirely from off-grid solar power.)

August 2007

In late August 2007 our quarterly electricity bill arrived with a huge 30% drop even though most of the power savings had only been in place for about 1 month before the meter reading! So almost all our electricity had been for powering these servers, and I'm now getting almost all the features that I was before for a tiny fraction of the energy and a lot less noise and heat in my office. On 1st September, at home all day (though on my own as it happens), less than 4kWh of mains electricity was used over a 24-hour period, compared to ~33kWh/day average for the previous quarter last year, thus nearly a 90% reduction in usage if this day proves anything like typical. Also, 4kWh/day is just about do-able by solar PV in the UK in our local area (there's a practical limit of about 4kWp of RE that the local electricity distribution network (EDF) will allow to be attached to one phase in a small domestic installation), and 4kWp should be able to generate 4kWh/day even on almost the darkest London winter days, so we could, with a grid-tie, be a net exporter overall, and on almost every single day too.

Now that I've stopped using expensive computing equipment as space heating, it will be interesting to see if our gas usage/bill goes up any in winter to compensate. (As of 2008/07 it would seem that we're up from ~9.1MWh/y to ~9.8MWh/y.) Even if it does, it's still better in money and CO2 terms to use mains gas (methane) to generate heat on site than to use electricity. If the house is too cold after this change then it will be worthwhile improving our insulation, not so before, which will be another good thing! (For the record, the August 2007 gas bill was ~£30 for 28 units (875kWh) for the quarter, ie ~10kWh/day, which is for heating and cooking, and is about 1/3rd the cost of electricity for those purposes.) On average a London 4kWp solar PV system might generate an average of ~10kWh/day (across the whole year) so we'd still be net overall consumers of energy between gas and electricity, especially in winter.

October 2007

From 1st October 2007 we have switched to the Ecotricity "New Energy Plus" tariff, so every single unit (kWh) we pull from the grid has been directly (or offset) generated by some mixture of wind/solar/hydro/wave "deep green" (mainly wind) for a 5% premium above our old rate from our current provider (EDF). Since we have cut our power usage by 80%--90% then we can easily afford to pay the extra 5% on the reduced bill! Interestingly, Ecotricity claims that 99% of their customers don't choose the 'Plus' tariff, so are presumably still very price sensitive, possibly overly so since trimming waste by 5% is likely to be easy for most of them. (As of 2007/09 we're using about 7.1kWh/day, and as of 2007/10 very slightly under 7kWh/day, which is somewhat higher than my target of 5kWh/day. On a sample January 2008 day with only me at home but typical consumption from running one load for the dishwasher and one for the washing machine, I observed between 5kWh and 6kWh use for the day. The BWEA figures suggest that ~13kWh/day (~390kWh/month) per household is typical for the UK (though government figures suggest ~9kWh/day), so we're pretty efficient already IMHO.)

March 2008

As of 2008/03/03 when we replaced our old fridge/freezer with an A+ rated one, I expect our gross consumption to fall to ~6kWh/day, and with the solar PV installed the previous week I expect our year-round net consumption to be ~5kWh/day, ie my original target for the whole house. On 2008/03/13 with the rest of my family away and almost no discretionary usage other than my laptop and a few cups of tea, I achieved a (nearly) 0kWh day, ie the import meter was reading the same kWh units at the end of the 24h as at the start. In reality I used about 0.8kWh more than was PV-generated.

For a tiny bit of perspective, one (now defunct) investment bank said that if all the computer monitors in its London head office were switched off every evening as of October 2007 then the bank would save as much as 500,000kWh per year worth £35,000 and reduce its carbon footprint by 21,000 kg of CO2. Total electricity use in the UK in 2006 was 382.5TWh, ie 382,500GWh, of which ~37% was coal, 36% gas, 18% nuclear and 4% renewables (see chapter 5 of the UK's 2007 Energy White Paper (EWP)). The Indian government target is to make available 1kWh/day to each household.


As of 2008/02/08, given the latest hike in rates and the fact that our usage is low enough that we are always in the first charging band (900kWh/quarter) which basically incorporates the standing charge for connection to the Grid, our electricity is now costing us about 16p/kWh, so ~£34/month. (~19p/kWh as of 2008/08.) So our dishwasher costs maybe 20p to run, the washing machine 10p each time, and the fridge over 30p/day; all extra incentives to trim a little further. (Ecotricity's "New Energy Plus" tariff charges a small premium over our regional supplier's (EDF's) standard rate to give us '100% green' power.)

November 2008

On the heating (gas) side we've added a total of maybe 26cm+ of loft insulation, put in thermostatic radiator valves (TRVs) where possible (and we're turning down or off each radiator when not in the room), put cardboard and/or foil behind radiators on exterior walls, put up a letter-box draught excluder and a door curtain for the front door, replaced some cracked/old/leaky (double-glazed) windows, put in an internal door curtain for our living room, and generally tried to trim heat demand. It seems from last month's figures that we may have nearly halved consumtion for a given number of heating-degree days, though it's difficult to tell yet.

Against that we have a newborn in the house this month which means more heat, washing and drying (and no chance to dry outside on the line).

December 2008

In 2008 we consumed just over 2MWh of electricity (<6kWh/d) and exported/generated 0.9MWh (solar PV, resulting in net consumption ~3kWh/d), and total mains natural gas consumption was well under 9MWh.

July 2009

We had our single-glazed front door replaced with a double-glazed unit, and we had an internal door fitted to our living-room, absent since move-in. We hope that both will reduce heating demand and improve comfort.

December 2009

For 2009 daily electricity consumption is down to a little over 5kWh (5 units), but on average the solar PV generates much more than we use, exporting a net excess of about 1MWh (1000 units) per year to the grid.

A new washing machine that allows cold and 30°C washes is probably saving us about 0.5kWh/day compared to last year, as is the latest energy-sipping incarnation of my Internet server.

In March--August 2009 (inclusive) the solar PV generated just over 2MWh with the increased output from the expanded system matching our expected consumption for the whole year.

Gas consumption for cooking, hot water and radiators has been a little over 6MWh for the year, down from ~9MWh/year previously.

As a result our net carbon footprint for the house is a little over 0.7tCO2/y, or maybe one eighth of that from before we started energy conservation, and possibly less than one-sixth of an average UK house's (based on 3.3MWh/y typical electricity use at 0.43tCO2/MWh thus 1.4t, and 18MWh/y gas at 0.19tCO2/MWh thus 3.4t, so 4.8tCO2/year total).


2011 was likely slightly carbon-negative.

March 2012

In March we were assessed to be a 'SuperHome' (number 119), with nominal carbon savings (ignoring our PV) of 62%.

May 2016

2016/05/14: with some adjustments to the off-grid battery logic and parameters all the Internet connection equipment (~12W) including FTTC (and the Loop energy monitor) are running off-grid all night leaving residual grid load ~80W according to the Loop energy monitor, of which ~40W is probably the fridge freezer. What portion of remainder is real (and used by which devices) and what is measurement error (ie bogus) is not clear. An 80W baseload does represent roughly one third of our annual electricity bill. Canidate always-on items are oven standby and clock, hob standby, MHRV fans, smart lighting controls, cordless phone, trailing mains socket/adaptor indicator lights, fridge-freezer (~40W), TV system standby on LIME controller ~0.5W, mains-powered 'smart home' monitoring/control devices (eg OpenTRV boiler control, though it should ~100mW and measures as zero), and night consumption by PV inverters. Occasional small extra loads of a few watts are expected from dishwasher and washing machine on delay waiting to start (and afterwards when done). Turning off all but the fridge and phone got reported overnight load to ~70W. So, out of ~80W night load breakdown of known parts is something like:

  • 40W : fridge/freezer
  • 8W: oven+hob
  • 2W: kitchen MHRV

See which electrical appliances use what at home.

Local Solar PV Generation

Daily PV Output Since 2008
In 2008 we started installing solar PV, and by 2010 had over 5kWp...

As of 2008/02/27 a grid-tie 1.29kWp solar PV system came on-line. This system should, on average throughout the year, generate ~2kWh/day (>4kWh/day in summer, but <0.4kWh/day mid-winter), and reduce imports by ~1kWh/day, ie remove the effect of the office consumption.

As of 2008/03, the system seems to be in balance with our baseload (ie no big/discretionary appliances nor the fridge/freezer running) with the import meter stationary at around 40W of PV generation, ie ~1kWh/day.

As of 2009/02/10 the grid-tie PV system on our roof has been expanded to nearly 4kWp which should generate nearly 3MWh/y compared to a consumption in 2008 of ~2MWh/y.

As of 2010/04 (after delays from equipment shortages from PV tariffs changing all over Europe) the PV system has been expanded again to over 5kWp which should generate nearly 4MWh/y compared to a (predicted) 2010 consumption of under 1.8MWh. (2010 final figures were 1.5MWh consumed, 3.5MWh generated; 2011 and 2012 were broadly similar though PV a little better.)

Off-site and 'Peak-Demand' Usage

I clearly don't just use energy in my home and study, but also in travel, etc...

Such as at my clients' offices, and in the various hosted servers that I use worldwide. As of November 2007 I'm trying to trim the power consumption of my offsite servers a little, for example minimising consumption of my London (UK) server at the time of UK electricity peak load (mainly 4pm--10pm local time) and above-mean carbon intensity, which reduces strain on the grid (a tiny amount!) and saves the CO2 from more carbon-intensive sources such as coal in 'peaking' and other generators at this time. For my Mumbai/Bombay (IN) Web server I'm deferring non-essential work 0700--2300 local time, in the spirit of the 2005/05/04 demand-curbing order of the Maharashtra Electricity Regulatory Commission. For the Sydney (AU) server I'm treating 0700--2300 (local) as 'peak' time, based on load graphs for nearby Victoria. For the Atlanta (Georgia, US) server, apparently summer (Apr--Sep) 2pm--10pm EDT is the peak load window, so I reduce my consumption then. I hope to handle the same global traffic with less CO2/day in future, but with similar low-latency response by keeping well-distributed mirrors.

Also, we avoid using the dishwasher and washing machine at home during winter peak-demand hours (4--9pm) so as to reduce overall CO2 generation.

(Rather embarrassingly, after many years of using our dishwasher, I had not paid any real attention to its 'quickwash' cycle until 2008/04, which seems to save 30% of the energy over our habitual prewash+wash setting. Time to make that change...)

Time-of-Day Metering Suggestion from South Africa to Curb Demand

As of January 2008 South Africa has been experiencing rolling blackouts (load-shedding) given various constraints on generation and transmission. It will likely be several years before this can be fixed.

One suggestion floating around is to put 'larger' (more than average) users on time-of-day metering where they have to pay more for energy at peak demand (and conversely may pay less at times of low demand).

I think that this is potentially an excellent suggestion, with the possibility of gently coercing the wasteful, idle and anti-social, while avoiding hurting the (energy) poor, so I wrote to my MP and MEPs 2008/02/02 (with the first couple of acknowledgements early on the Monday 2008/02/04):

Re: Reducing CO2/GHG from electricity generation

[Might you suggest] implementing a similar rule to that currently being touted in South Africa to help economise on electricity usage and deal with their acute shortages and load-shedding black-outs?

It is nice and simple, doesn't penalise the (energy) poor, and has analogues from deliveries of other constrained resources such as water.

Simply that any user of electricity that exceeds 500kWh/month gets time-of-day metering, which is not necessarily more expensive, but rewards those users for avoiding times of peak demand. In effect it is a far more dynamic alternative to schemes such as 'Economy 7'.

It will also tend to encourage more economy in winter when electricity generation is likely to be more carbon intensive.

We have the technology for time-of-day metering already.

If you wish, you can read about my family's efficiency efforts which have brought us to about 210kWh/month:

A couple of the MEPs have responded that they will forward my suggestion to the relevant European Commissioner.

Syed Kamall, Conservative MEP for London responded:

Your ideas are certainly very interesting and I will certainly put them to energy experts in the European Parliament and in London.

However, we should also note that the acute shortages and black-outs being experienced in South Africa are partly due to Eskom pricing energy so cheaply (for political reasons) that there was no incentive to encourage energy efficiency and conservation. Eskom's low tariffs have also driven away much needed potential local and foreign private investors.

Let's hope that Eskom and the South African government have learned the lessons and move towards more energy efficiency.

I agreed that while the causes/motivations aren't identical, the solutions might be much more nearly so.

On 2008/04/12 I received a letter from the European Commission c/o John Bowis OBE MEP (London) that notes that:

The reduction of electricity consumption through the use of TOD metering ... is indeed one of a series of differentiated methods applicable to energy saving.

The implementation of innovative measures aimed at reducing the energy demand of the houses (and hence CO2/GHG produced from electricity generation) without penalizing comfort is one of the essential elements in energy management policy. The projects on Eco-building and Concerto are examples in this field and more information can be found in their respective websites: [] and

See the early 2008 proposed regulations and the Homeflex study from South Africa.

Interestingly, an informal on-line poll of usage per month, in a blog read by a worldwide audience including the UK and US, indicated as of 2008/02/04 that the modal monthly kWh is around 500 to 600, which suggests that that would be a good target threshold to maximise effect. It might be more politic to start with a much higher value and ratchet it down to avoid bringing more than 25% to 50% of users into its net involuntarily and suddenly (and expensively in terms of meter changeovers). When the poll closed in March the results as follows suggested that a ~500kWh/month threshold would still be plausible:

How much electricity do you use each month?
Less than 300kWh18
300 to 600kWh26
600 to 900kWh18
900 to 1200kWh8
1200 to 1500kWh12
1500 to 1800kWh2
More than 1800kWh11

From Net-Zero Electricity to Negative-Carbon

Being net-zero for electricity year-round and using the grid to 'store' excess until needed (eg in summer for winter, and in the day for the night) is not the same as being zero-carbon...

An excess unit of solar-PV-generated electricity exported in summer daytime probably displaces efficient CCGT (gas) burn which is the least carbon-intensive fossil-fuel generation mechanism/fuel, but a unit imported when the sun is down, especially at winter peak demand, will have used in part a more carbon-intensive generation method and fuel, so zeroing the electricity use has not zeroed the carbon generation. This is especially likely given the "Large Combustion Plants Directive" (LCPD, 2001/80/EC, PDF on the Europa website) in force from 2008/01/01, limiting the total number of running hours of 'opted-out' fossil-fuel generators to 20,000 across the 8 year period to 2015. It is not clear how each plant will be run, but probably some will generate only at times of peak prices/demand to maximise profit.

If we were completely off-grid then we'd have to cover each day's use by our own RE generation, probably mainly from solar PV in urban London. For our 7kWh/day to be covered in mid-winter we'd need 7kWp of solar PV (plus a least a day's storage in batteries, or several days' for bad weather) compared to the 4kWp we'd need for year-round net-zero electricity. In fact, storage in batteries probably loses ~20%, so we'd better allow 8kWp, ie double our net-zero-electricity value nameplate peak capacity. And adding the storage to cover several days' bad weather redoubles costs, bringing us to maybe £100k to lose/zero all grid dependence and thus to erase the carbon footprint for our electricity.

However, we can then do better than zero-electricity and zero-carbon, by reconnecting to the to the grid in summer to export any excess, maybe 80%. We become a little solar PV generating station for the grid all bar mid-winter. We might not even be allowed to export ~30kWh/day on our existing single-phase connection, especially given that there is a current limit of ~16A peak. However, if (for example) we optimised panel orientation for mid-winter output and/or to spread generation throughout a summer day (eg with panels facing east and west rather than all south) possibly with some simple mechanical adjustments by season, then our exports would reduce CO2 generation for other grid users, and thus be 'negative carbon' from our point of view.

Our 8kWp system might produce ~4MWh/year excess to our needs, ie exportable, (of maybe 6--7MWh/year available generated power total), which we might sell in London to the local distribution network (EdF) for ~£500/year including ROCs, etc. So we wouldn't get rich that way (200 year payback cf 25 year equipment life), but we would be 'negative carbon' for electricity. This doesn't cover our space/water heating (CH/DHW) requirements.

It would be great to get overall energy efficiency to CSH (Code for Sustainable Homes) Code Level 4 or better as I understand it (44% reduction in energy use over 2006 Part L building regulations, based on a gas-heated home, due to be mandatory for new build by 2013 anyway), though currently with ~10MWh/year electricity and ~9MWh/year gas consumption vs an apparent UK average of 20+MWh/year, we may in effect already be achieving the implied energy savings in practice. (For example, we have the central heating thermostat at ~18°C during the day and a at least a couple of °C lower still overnight vs the 'standard' 21°C SAP (Standard Assessment Procedure) rating.)

As of late November 2007 I sent an enquiry to the National Grid to see if a G83/1 (11kW/3-phase) inverter would be useful to them as part of their standing/frequency reserve used to help maintain grid stability, for example when a generator drops out unexpectedly or load spikes unexpectedly. With a relatively small (6kWh) battery bank such a grid-tie inverter system might be able to cut in very fast (within one cycle) and run for 30 minutes in case of falling frequency or other stresses such as mid-winter peak demand, even when the sun is down, which seems to meet some of their technical requirements (eg plant must be able to start delivery within 2 minutes of instruction, have a run up or run down rate of 25MW per minute, and must have capability to sustain output for at least 15 minutes) but fails to meet another by more than three orders of magnitude (that the plant can be instructed in blocks of at least 50MW). I'm all in favour of explicitly contributing to grid stability as part of a distributed generation system, and providing reserve might earn some fees.

As of 2009/02/10 the grid-tie PV system on our roof has been expanded to over 3kWp and our consumption reduced sufficiently that we will be net exporters over the course of a year, possibly with sufficient margin of excess export that we can expect to be carbon-neutral (or negative) for electricity.

2011 was in fact probably slightly carbon-negative and we continue to work on efficiency.


Managing solar PV with an Energy Services Company...

To deploy a really large solar thermal/PV system effectively in all regards, including financially, it makes sense to have the capital and other expenditure handled by an independent financial entity with, for example, VAT registration to avoid 'losing' 17.5% down the cracks on expenditure, etc.

This company would be an ESCo (Energy Services Company), and there would have to be a little care taken (and maybe some negotiation with the tax authorities) to ensure that, for example, having a large chunk of the roof area covered with collectors does not somehow undermine full capital gains tax relief on the property. In particular, that is likely to mean the household paying the ESCo at least a transparent commercial rate for energy provided. Presumably on moving out the ESCo would have to be sold at fair commercial value to the new home owners, but would that count towards stamp-duty thresholds for example?

As of early 2008 my consultancy company is going carbon neutral for the office at home with a modest ~1kWp scheme, and will act as its own ESCo.


Metered Mains Energy Use/Generation

Before and after starting conservation, at the end of 2007, more than halving bills from 10,000 units per year of electricity and 9,000 units per year of gas...

Before Starting Energy Conservation

We moved into our current house 2004/06/14. From 2004/06/14 to 2007/08/21 we used 30705 units of mains electricity (~31MWh) over 38 months, ie ~27kWh/day, or ~10MWh/year electricity ie ~4.3tCO2/year. From 2004/06/14 to 2007/08/21 we used 869 units of mains gas (28 units since 2007/05/17, and 865 units (~27MWh) over the 36-month period 2004/10/21 to 2007/10/17, with an annual HDD12* of ~1160) ie ~25kWh/day, or ~9MWh/year gas ie ~1.7tCO2/year.

Thus our total primary domestic energy consumption (gas and electricity) was responsible for ~6t CO2 per year for two adults (and then lately two children), or ~3t CO2 per year per adult.

After Starting Energy Conservation

Monthly Gas Consumption Since 2007 Monthly Electricity Consumption Since 2007 (gross) Yearly Electricity Consumption Since 2005 (gross) Yearly Gas Consumption Since 2005 Yearly E+G Carbon Footprint

From 2007/08/21 to 2007/12/31 (132 days) we used 936 units (kWh) of electricity ie ~7.1kWh/day, and 126 units (mains methane/natural) gas ie 3963kWh or 30kWh/day. That included:

  • Sep electricity 282 units (7.1kWh/day) + 10.2kWh/day gas over 40 days including end of Aug (*HDD12=13)
  • Oct electricity 206 units (6.6kWh/day) + 20.3kWh/day gas over 31 days (HDD12=53)
  • Nov electricity 205 units (7.3kWh/day) + 42.8kWh/day gas over first 28 days (HDD12=134)
  • Dec electricity 243 units (7.3kWh/day) + 52.5kWh/day gas over 33 days (HDD12=192; >60kWh/day coming up to Christmas)

(In 2007 up to 2007/05/09 used approx 4040kWh electricity (estimated), 2007/05/09 to 2007/08/21 used 1992kWh (from bill), 2007/08/21 to 2007/12/21 used 936kWh (from above), to a total of 6968kWh for 2007.)

(Assuming 1 unit (the meter says 100cuft) of gas as shown on our meter = 31.5kWh, with conversion figures taken from 2004/07 bill and verified with 2007/10 bill and 11.1kWh/m^3 after our meter change 2009/06/04. *HDD12 is 'heating degree days' for EGLL (Heathrow Airport, aka LHR, nearby) for a base temperature of 12°C, lower than the usual 15.5°C to better reflect our usage, c/o

Note that figures for kWh generated come from our OFGEM-approved meters, whereas the per-day log files are the slightly-less-generous outputs from the grid-tie inverters, thus there are small differences (~5% as of end-2009).

By Year

2008 Daily PV Output


2008 data (kWh: e used 2036, e gen -901, g 8883) and discussion...

See the daily PV generation logs for 2008 as .csv and .png chart.

MonthConsumed kWh (/d)Generated -kWh (/d)Consumed kWh (/d)PeriodNotes
2008/01 214 (6.9/d) 1418 (45.7/d) 20071231-20:38-GMT to 20080131-20:51-GMT (31d) Gas use >60kWh/day at the end of the month. No microgeneration (other than a tiny off-grid PV system). HDD12=140.
2008/02 195 (6.7/d) -4 (-1.6/d) 1449 (50.0/d) 20080131-20:51-GMT to 20080229-20:06-GMT (29d) Gas use >60kWh/day at various points. Import meter unratcheted thus able to run in reverse, so actual import readings totalling 191kWh shown with the 4kWh of the PV generation meter added back on. PV system operational for only the last 2.5d approx of the period from 2008/02/27 ~13:00 with first ~0.4kWh missed in the logs (.csv). See the projected outputs from the PV system. HDD12=172.
2008/03 187 (6.0/d) -70 (-2.2/d) 1323 (42.7/d) 20080229-20:06-GMT to 20080331-18:36-GMT (31d) Import meter unratcheted, so reading of 117kWh shown with 70kWh of the PV generation added back on. PV generation logs (.csv).
Replacement of fridge/freezer with A+ model reduced electricity consumption by ~1kWh/day. HDD12=157.
2008/04 165 (5.5/d) -108 (-3.6/d) 882 (29.4/d) 20080331-18:36-GMT to 20080430-20:21-GMT (30d) Import meter unratcheted, so reading of 57kWh shown with 108kWh of the PV generation added back on. PV generation logs (.csv). HDD12=105.
2008/05 146 (4.7/d) -135 (-4.4/d) 290 (9.3/d) 20080430-20:21-GMT to 20080531-19:18-GMT (31d) Import meter unratcheted, so reading of 11kWh shown with 135kWh of the PV generation added back on. PV generation logs (.csv).
HDD12=22. The central heating was turned off 2008/05/14.
2008/06 146 (4.9/d) -160 (-5.3/d) 264 (8.8/d) 20080531-19:18-GMT to 20080630-20:05-GMT (30d) Import meter unratcheted, so reading of -14kWh shown with 160kWh of the PV generation added back on. PV generation logs (.csv). HDD12=6.
I estimate that our gas use for DHW is now a little over 6kWh/day, and cooking is the only other use at this time of year.
The electricity bill for the first six months of the year was just over £80, even on the premium 'all renewables' tariff.
2008/07 139 (4.5/d) -161 (-5.2/d) 204 (6.6/d) 20080630-20:05-GMT to 20080731-23:59-GMT approx (31d) Import meter unratcheted, so reading of -22kWh shown with 161kWh of the PV generation added back on. PV generation logs (.csv). HDD12=3.
Not present to take reading due to holiday, so estimated by subtracting 4kWh from the import and 6.5kWh from the PV generation readings of 20080802-13:30-GMT (no gas was used at all while we were away).
2008/08 147 (4.7/d) -104 (-3.3/d) 230 (7.4/d) 20080731-23:59-GMT to 20080901-06:48-GMT (31d) Import meter unratcheted, so reading of ~44kWh shown with ~104kWh of the PV generation added back on. PV generation logs (.csv). HDD12=1.
Adjustments for end-of-July reading taken into account.
2008/09 152 (5.1/d) -85 (-2.8/d) 271 (9.0/d) 20080901-06:48-GMT to 20080930-17:28-GMT (30d) Import meter unratcheted, so reading of ~67kWh shown with ~85kWh of the PV generation added back on. PV generation logs (.csv). HDD12=11.
Electricity bill for Jul/Aug/Sep just over £15 and <1kWh/day net!
2008/10 160 (5.2/d) -53 (-1.7/d) 529 (17/d) 20080930-17:28-GMT to 20081031-19:47-GMT (31d) Import meter unratcheted, so reading of ~107kWh shown with ~53kWh of the PV generation added back on. PV generation logs (.csv). HDD12=83, cf HDD12=53 last year, with heat demand 17kWh/day vs 20kWh/day last year, which implies nearly doubled space heating efficiency.
Heating turned on this (fairly chilly) month, and we had had a couple of thermostatic radiator valves (TRV) fitted ready, with more due next month. The boiler broke down shortly after its service, so heating was erratic for a while.
2008/11 191 (6.4/d) -18 (-0.57/d) 828 (28/d) 20081031-19:47-GMT to 20081130-18:30-GMT (30d) Import meter unratcheted, so reading of ~173kWh shown with ~18kWh of the PV generation added back on. PV generation logs (.csv). HDD12=128, cf HDD12=134 last year, with heat demand 28kWh/day vs 43kWh/day last year, again implying significantly improved efficiency.
We had a new arrival in the household this month, thus extra demand for heat, washing, drying (that couldn't go outside on the line of course), etc! Green tip: have your babies in the spring...
2008/12 210 (6.8/d) -17 (-0.54/d) 1194 (39/d) 20081130-18:30-GMT to 20081231-18:09-GMT (31d) Import meter unratcheted, so reading of ~194kWh shown with ~18kWh of the PV generation added back on. PV generation logs (.csv). HDD12=233, cf HDD12=192 last year, with heat demand ~39kWh/day vs ~53kWh/day last year, and never over 60kWh/day.
We managed to meet our gas (heating energy) budget for the winter taking us up to <8.9MWh for the year.
The electricity bill to the end of this month was a little over £70, so ~£170 for the year, even at a premium 'green' rate, and the gas bill ~£280 to the year end: in total less than half the 'typical' £1000+.
2008 total (Jan--Dec) 2036 (5.6/d) -901 (-2.9/d) 8883 (24/d) 20071231-20:38-GMT to 20081231-18:09-GMT (366d) 1.29kWp grid-tied solar PV system operational from 2008/02/27, ie 57.5d after year start.
See the daily PV generation logs for 2008 as .csv and chart.
HDD12 for the year totals 1061.
(DUKES 2006 figures that suggest average consumption per GB household of natural gas is ~18MWh/y (400TWh/22M) and electricity is ~4.5MWh/y (118TWh/26.5M, though modal UK use is a little over 3MWh/y), so we are comfortably less than half of average for each.)
kgCO2 875kg (2.4/d) -387kg (-1.1/d) 1688kg (4.6/d) ~2.2tCO2 (~5.9kgCO2/d) gas/electricity emissions total for 3 people (4 from mid-November), or about 2/3rds reduction for the household from only a year or two ago, based on 0.43kgCO2/kWh for electricity (netted) and 0.19kgCO2/kWh for gas. This SAP energy performance certificate computed 2009/03 estimates 1.9tCO2.
(Cf 2007/8 5.2t/person total footprint in China, 8.6t Western Europe, 19.4t US. David JC MacKay author of Sustainable Energy - Without the Hot Air in private communication suggests that in the UK public/government services alone emit ~5.5t/person/y as of 2008.)
Note that we haven't flown this year (nor indeed for some years), we don't own a car and travel almost entirely by (usually-electrified) public transport (we drove ~80 miles in a hire/rental car in 2008).

During 2008 gas consumption for cooking and water heating is ~8kWh/day (from summer when there is no space-heat demand and we are not on holiday), therefore totalling ~3MWh for the year. This leaves ~6MWh as space heating, with 1061 heating-degree-days @ 12°C, ie about 5.7kWh/HDD12. By contrast, the equivalent for the first 3 months of 2009 is ~3.9kWh/HDD12, which implies 30%--40% improved heating efficiency as previously inferred. And indeed, given that 2008 benefited from efficiency measures at the end, comparing with the first 3 months of 2008 at ~7.4kWh/HDD12 more clearly indicates a near-doubling of efficiency over a year.

Our total exterior wall / ground floor / roof area is ~164m^2, so a heating demand of 3.9kWh/HDD12 (162W/K) as at the start of 2009 corresponds to an effective U-value of approx 1W/K/m^2 overall.

During the recent air-leakage and SAP investigation one of the parameters estimated was the U-value of the large blank north wall, at around 0.8W/m^2/K, so over its ~40m^2 that's 32W/K or ~0.77kWh/K/day, or ~0.77kWh/HDD in the terms above, or about 20% of total heat loss. Modern building regs require a U-value of <0.4W/m^2/K, and retrofitting that wall to meet or exceed that would maybe save 10% or 0.6MWh/y in a harsh winter. Similarly improving the other exterior walls (~50m^2 - window area) could save that full 20% or ~1MWh/y, or half my 2009 target. This broadly agrees with the SAP calculations which suggest that ~30% of heat is being lost through the walls currently (including ventilation). Window, ground floor and ventilation losses all tie at about 20%. In general it would be good to exceed building regs by a 'best practice' margin of (say) 25% if it can be done to maximise the return given the disruption.

As of our June 2009 holiday the house base electricity consumption, in our absence with essentially just the fridge/freezer and server/ADSL running, was just over 2kWh/day. When I am here on my own (and my family is away) I tend to use about 0.5kWh/day extra.

We had a new electricity meter installed 2008/02/11 since the old one might have been unsafe, but the new one was unratcheted and thus ran backwards when we exported/"spilled" to the grid which apparently causes various accounting and reporting traumas, so we had yet another change (to an import/export pair) 2009/09/21!

The new import/export meter pair was installed in under an hour (unfortunately at solar noon, thus losing a prime hour's generation!). The meters are digital (mechanical rather than electronic) so we now have three digital kWh readouts: generation, import and export. Note that generation - (import + export) is whatever we consume internally.

The gas company's estimate of our consumption from 2009/01/02 to 2009/06/29 though a meter change, etc, is 3714.6kWh. My estimate from 2009/01/01 to 2009/06/30 is 3744kWh, which is pretty-well spot on given the two days' difference.

2009 Daily PV Output


2009 data (kWh: e used 1942, e gen -2958, g 6201) and discussion...

See the daily PV generation logs for 2009 as .csv and .png chart.

MonthConsumed kWh (/d)Generated -kWh (/d)Consumed kWh (/d)PeriodNotes
2009/01 214 (6.9/d) -23 (-0.75/d) 1285 (41/d) 20081231-18:09-GMT to 20090131-18:05-GMT (31d) Import meter unratcheted, so reading of ~191kWh shown with ~23kWh of the PV generation added back on.
HDD12=266 vs 140 last January (temperatures ~1.5°C lower) but ~10% less heating/gas demand, so roughly doubled heating efficiency.
Heating is now set to come on for about two 15-minute slots during the night if the house temperature drops below ~15°C since our little one's thermoregulation still needs some help: this keeps the night-time temperature more stable than it might otherwise be on very cold nights. That's somewhat less than we have had in the previous couple of months, and it doesn't seem to increase gas consumption enormously. This is a frugal form of 'setback' temperature.
I note that we used exactly the same amount of electricity as last January, so an efficient fridge seems to have exactly compensated for a newborn!
2009/02 178 (6.3/d) -73 (-2.6/d) 1046 (37/d) 20090131-18:05-GMT to 20090228-18:03-GMT (28d) Import meter unratcheted, so reading of ~105kWh shown with ~73kWh of the PV generation added back on.
PV system expanded (tripled in size) early this month, nominally making us net exporters/generators over the course of a year.
HDD12=203 vs 172 last February, but much less gas used.
2009/03 177 (5.7/d) -254.5 (-8.2/d) 778 (25/d) 20090228-18:03-GMT to 20090331-16:40-GMT (31d) Import meter unratcheted, so reading of -77kWh shown with ~255kWh of the PV generation added back on.
Upgraded to more efficient washer/dryer that can wash in cold/unheated water which may thus save us ~0.5kWh/day depending on laundry soiling/mix.
HDD12=140 vs 157 last March, but disproportionately less gas used.
2009/04 149 (5.0/d) -356 (-11.9/d) 265 (8.8/d) 20090331-16:40-GMT to 20090430-20:23-GMT (30d) Import meter unratcheted, so reading of -207kWh shown with 356kWh of the PV generation added back on.
Central heating has been off for most of this (unusually warm) month.
HDD12=57 vs 105 last April.
2009/05 147 (4.7/d) -458 (-14.8/d) 226 (7.3/d) 20090430-20:23-GMT to 20090531-17:33-GMT (31d) Import meter unratcheted, so reading of -311kWh shown with 458kWh of the PV generation added back on.
HDD12=25 vs 22 last May.
2009/06 130 (4.3/d) -485 (-16.2/d) 144 (4.8/d) 20090531-17:33-GMT 20090630-18:06-GMT (30d) Import meter unratcheted, so reading of ~-356kWh shown with 485kWh of the PV generation added back on.
HDD12=8 vs 6 last June.
(Adjusting for the 7 days that we were on holiday, with base electricity consumption of 2kWh/d for fridge/freezer and Internet server, daily gross electricity consumption while we were here was 5.0kWh/d and gas 6.3kWh/d, ie down from a baseline of ~8kWh/d gas for cooking/washing last year.)
The gas meter was changed to a metric (m^3) model this month, with much finer resolution than before, nominally down to 0.001m^3 but with some effort probably 0.0001m^3 or about 1Wh (at ~11.1kWh/m^3)...
2009/07 163 (5.1/d) -447 (-14.0/d) 203 (6.3/d) 20090630-18:06-GMT to 20090801-13:26-GMT (32d) Import meter unratcheted, so reading of -284kWh shown with 447kWh of the PV generation added back on.
HDD12=1 vs 3 last July.
Single-glazed front door replaced with double-glazed unit, and internal door fitted between living-room and corridor.
(I wasn't at home for the time-honoured month-end meter-reading ceremony, so there is just short of an extra day's flows, with the extra gas being ~0, electricity consumption ~2kWh, and generation ~7kWh.)
I'm not sure why our electricity consumption is apparently up a little, though July was fairly damp and dull.
2009/08 143 (4.8/d) -374 (-12.5/d) 166 (5.5/d) 20090801-13:26-GMT to 20090831-18:00-GMT (30d) Import meter unratcheted, so reading of -231kWh shown with 375kWh of the PV generation added back on.
HDD12=2 vs 1 last August.
2009/09 154 (5.1/d) -256 (-8.5/d) 189 (6.5/d) 20090831-18:00-GMT to 20090930-17:03-GMT (30d) Unratcheted import meter replaced during the month with an import/export meter pair (both initially reading 1.6kWh). We lost about 1kWh of generation while our mains was disconnected, and thus our grid-tie was off-line, while the replacement took place. Up until the meter replacement the old meter reading was -84kWh, and generation was 182kWh, thus making for a net consumption of 99kWh for the month until then. Since the replacement we exported 51.5kWh of the generated 73.3kWh, and imported 33.3kWh, making net consumption 55.1kWh. (That is, net consumption = generation - export + import.) We directly consumed in the house 30% ((gen-exp)/gen) of our PV generation. Or, another way, we avoided importing 40% ((con-imp)/con) of our consumption. Total consumption for the month was 154kWh.
HDD12=7 vs 11 last September.
2009/10 155 (5.0/d) -126 (-4.1/d) 300 (9.7/d) 20090930-17:03-GMT to 20091031-17:20-GMT (31d) Meter readings: electricity generation 125.9kWh, export 91.4kWh, import 120.2kWh (thus net consumption (I+G-E) 154.7kWh); gas 27.071m^3 (so 300kWh @ 11.1kWh/m^3).
Replaced 24x7 Internet laptop/server with entirely off-grid-solar-powered SheevaPlug (@~4W) which I believe has caused mains electricity consumption to drop.
HDD12=32 vs 83 last October. (We started using central heating mid-month.)
2009/11 161 (5.4/d) -61.5 (-2.1/d) 558 (19/d) 20091031-17:20-GMT to 20091130-17:26-GMT (30d) Meter readings: electricity generation 61.5kWh, export 36.1kWh, import 135.7kWh (thus net consumption (I+G-E) 161.1kWh); gas 50.245m^3 (so 558kWh @ 11.1kWh/m^3).
Battery got very low so SheevaPlug back on mains for some of this wet and windy month.
HDD12=78 vs 128 last November (cumulative HDD=812, gas 5160kWh this year; HDD12=828, gas 7689kWh last year).
2009/12 169 (5.5/d) -42.7 (-1.4/d) 1043 (34/d) 20091130-17:26-GMT to 20091231-18:07-GMT (31d) Meter readings: electricity generation 42.7kWh, export 21.0kWh, import 147.4kWh (thus net consumption (I+G-E) 169.1kWh); gas 93.919m^3 (so 1043kWh @ 11.1kWh/m^3).
SheevaPlug back on mains for getting on for half of this month.
HDD12=250 vs 233 last December.
2009 total (Jan--Dec) 1942 (5.3/d) -2958 (-8.1/d) 6201 (17/d) 20081231-18:09-GMT to 20091231-18:07-GMT (365d) Electricity consumption down ~5% on last year, generation up ~3x; now ~50%/1MWh net exporters. Gas consumption down ~30% on last year.
HDD12 for the year 1068 (vs 1061 for 2008).
kgCO2 835kg (2.3/d) -1272kg (-3.5/d) 1178kg (3.2/d) ~0.74tCO2 (~2.0kgCO2/d) gas/electricity emissions total for 4 people, based on 0.43kgCO2/kWh for electricity (netted) and 0.19kgCO2/kWh for gas.
Note that we haven't flown this year again, we still don't own a car and travel almost entirely by (usually-electrified) public transport (we drove ~80 miles in a hire/rental car in 2009).

Analysis of 2009 figures shows ~1MWh electricity net export (~0.44tCO2 displaced elsewhere on the grid), and ~6MWh gas consumption (~1.2tCO2 direct emissions), giving a net primary-energy CO2 footprint for the house of ~0.74tCO2, down from 2.2tCO2 in 2008 and 6tCO2 in preceding years. There were two of us here initially; now there are four.

(With the UNFCCC 0.67t/MWh figure allowed for green electricity generation, applied to all our electricity consumption and generation, the household net for the year including gas is just under 0.5tCO2, so exporting an extra ~1MWh/y on top would make the house zero-carbon.)

Note that we start/run about 25% of our gross electrical consumption/loads (dishwasher and non-cold washing-machine cycles) during the night ~1am to ~6am when grid carbon intensity is lowest. Our solar PV in contrast is generating and exporting mainly when grid carbon intensity is highest. So chosing a flat intensity figure should be reasonable and conservative.

In 2008 I estimated our daily cooking and hot-water gas consumption at 8kWh/day. In 2009 I believe that we've reduced that to ~6kWh/day ie ~2MWh/y (partly due to me taking fewer, shallower, cooler baths) which has the side-effect of reducing our apparent heating-efficiency gains!

Given the 2009 HDD12 of 1068 this indicates ~3.8kWh/HDD12 heating efficiency for 2009, ie roughly in agreement with the estimate for the start of 2009, once the reduced daily cooking/DHW 'baseline' consumption is allowed for.

My aim for 2010 is to increase solar PV generation to export ~2MWh/year net (thus deferring an extra ~430kgCO2/year on the grid), and improve insulation and reduce DHW use to save ~1.5MWh/year gas (~290kgCO2), to get us approximately carbon-neutral for primary energy, ie achieve a "zero-carbon home".

A somewhat longer-term goal is to replace the gas combi with a heat-pump once the year-round CoP, and the grid's carbon intensity, will result in less CO2 emitted, which implies a CoP > 2.5 at 0A/55W (0°C ambient, 55°C DHW/rads).

See the 2009/2010 carbon-footprint graph. iMeasure used the 2007 emission factor guidance from DEFRA: gas is 0.206kgCO2/kWh and electricity is 0.523kgCO2/kWh (5-year rolling average).

2010 Daily PV Output


2010 data (kWh: e used 1554, e gen -3547, g 5784)...

See the daily PV generation logs for 2010 as .csv and .png chart.

MonthConsumed kWh (/d)Generated -kWh (/d)Consumed kWh (/d)PeriodNotes
2010/01 181 (5.9/d) -42 (-1.4/d) 1233 (40/d) 20091231-18:07-GMT to 20100131-17:30-GMT (31d) Meter readings: electricity generation 42.3kWh, export 25.2kWh, import 164.3kWh (thus net consumption (I+G-E) 181.4kWh); gas 111.070m^3 (so 1233kWh @ 11.1kWh/m^3).
SheevaPlug on grid (on mains electricity) most of this month as I was conserving off-grid battery in case we had power outages in the severe weather and needed lighting, etc.
HDD12=316 vs 266 last January (mean temperatures 2.5°C or more below 1971--2000 normal), but our gas and electricity consumption was down on last year. I am inclined to attribute most of this reduction to the new internal door, and the blocking of some major unplanned ventilation (ie air leaks), and a little to our new front door and behaviour changes.
According to i-measure our primary-energy emissions peaked for two weeks this month (and probably for the whole winter) at 61kgCO2/wk from gas and 21kgCO2/wk from imported electricity, ie ~20kgCO2/wk for each of the four of us.
2010/02 141 (5.0/d) -76 (-2.7/d) 920 (33/d) 20100131-17:30-GMT to 20100228-17:02-GMT (28d) Meter readings: electricity generation 75.8kWh, export 50.8kWh, import 116.0kWh (thus net consumption (I+G-E) 141.0kWh); gas ~83m^3 (so 920kWh @ 11.1kWh/m^3).
SheevaPlug on grid (on mains electricity) all month through there was a near-death experience mid-month when the battery LVD was about to switch off but we had the sunniest day so far this year and filled the battery!
HDD12=232 vs 203 last February (the BBC says that the UK has had its coldest winter in over thirty years and the coldest February since 1991) but our gas and electricity consumption was down again on last year.
2010/03 154 (5.0/d) -207 (-6.7/d) 694 (22/d) 20100228-17:02-GMT to 20100331-18:02-GMT (31d) Meter readings: electricity generation 207.4kWh, export 165.7kWh, import 112.6kWh (thus net consumption (I+G-E) 154.3kWh); gas ~63m^3 (so 694kWh @ 11.1kWh/m^3).
We had the heating off entirely for a couple of days towards the end of March, and dried washing on the line outside a few times.
HDD12=165 vs 140 last March, but still less gas used!
2010/04 126 (4.1/d) -378 (-13/d) 213 (7.1/d) 20100331-18:02-GMT to 20100430-17:07-GMT (30d) Meter readings: electricity generation 378.1kWh, export 329.2kWh, import 76.8kWh (thus net consumption (I+G-E) 125.7kWh); gas ~19m^3 (so 213kWh @ 11.1kWh/m^3).
We had the heating off from the start of the month, and dried lots of washing on the line!
Another grid-tied PV system on the roof went live right at the end of the month, which should get the house very close to zero-carbon.
HDD12=82 vs 57 last April, and less gas used!
2010/05 129 (4.2/d) -564 (-18/d) 180 (5.8/d) 20100430-17:07-GMT to 20100531-18:15-GMT (31d) Meter readings: electricity generation 563.5kWh, export 505.1kWh, import 70.6kWh (thus net consumption (I+G-E) 129.9kWh); gas ~16m^3 (so 180kWh @ 11.1kWh/m^3).
Export exceeded import mid-month for the new DNO meter pair installed 2009/09.
Overnight import ~0.7kWh/d if dishwasher and washing machine not run, else ~2.3.
During a run of 4 days of clear skies we hit a generation record of 30.38kWh with similar amounts ~29kWh surrounding it, with peak output ~3.5kW. On other days peaks of over 4kW have been seen (vs nominal 5.16kWp of panels, limited by 4.5kWp of inverters). Total generation since the first PV was put in exceeded 5MWh this month, probably making us 'electricity neutral' since then, with generation on the first meter, ie excluding the newest PV installation, about to pass 5MWh itself in the first few days of next month.
HDD12=66 vs 25 last May, and less gas used again implies DHW conservation.
2010/06 132 (4.4/d) -650 (-22/d) 156 (5.2/d) 20100531-18:15-GMT to 20100630-17:56-GMT (30d) Meter readings: electricity generation 649.8kWh, export 578.6kWh, import 60.4kWh (thus net consumption (I+G-E) 131.6kWh); gas ~14m^3 (so 156kWh @ 11.1kWh/m^3).
A peak of 32.65kWh was generated on 2010/06/03. The longest day was not a clear day unfortunately, so not a record!
Note that gas consumption was very slightly up on this month last year, due to holiday dates.
HDD12=8 vs 8 last June.
2010/07 112 (3.6/d) -598 (-19/d) 133 (4.3/d) 20100630-17:56-GMT to 20100731-16:57-GMT (31d) Meter readings: electricity generation 598.4kWh, export 543.0kWh, import 57.0kWh (thus net consumption (I+G-E) 112.4kWh); gas ~12m^3 (so 133kWh @ 11.1kWh/m^3).
Used 4kWh/d @ 7p/kWh thus £2 for week's (UK) holiday this month, including immersion DHW in our chalet. Thus total electricity consumption from GB grid actually ~140kWh, ie 4.5kWh/d!
Note that the last few miles by taxi of our ~150 mile holiday trip each way cost as much as the rest by (electrified) train(s).
This month our living room was superinsulated with aerogel, so should contribute to lower heating demand over winter.
HDD12=0 vs 1 last July.
2010/08 124 (4.0/d) -430 (-14/d) 148 (4.8/d) 20100731-16:57-GMT to 20100831-17:18-GMT (31d) Meter readings: electricity generation 429.6kWh, export 374.6kWh, import 69.2kWh (thus net consumption (I+G-E) 124.2kWh); gas ~13m^3 (so 148kWh @ 11.1kWh/m^3).
Passed 6.5MWh PV generation (0.5MWh from the newest 1/4) early in the month.
So far this year, electricity consumption ~1099kWh, generation ~2954kWh, and gas consumption ~3677kWh, ie maybe a net electricity CO2e footprint of -0.798t (@0.43kg/kWh) and +0.699t from gas (@0.19kg/kWh), ie a small footprint of ~-0.1t. Assuming for the rest of the year (4M/122d) 4.5kWh/d electricity consumption (ie 549kWh), and generation 4/3 that for the same period last year (ie 648kWh), that leaves us with a further -0.043t footprint. To reach zero-carbon we could use that -0.141t for gas heating, ie 742kWh, though after ~5kWh/d gas for DHW/cooking that leaves only ~1kWh/d for central heating! Given that we used 2090kWh for the last four months of 2009, then unless this is an exceptionally mild winter we won't reach zero, quite, but should miss it by less than 0.3tCO2.
HDD12=4 vs 2 last August, and the fourth dullest August on record.
2010/09 141 (4.7/d) -318 (-11/d) 180 (6.0/d) 20100831-17:18-GMT to 20100930-17:18-GMT (30d) Meter readings: electricity generation 317.8kWh, export 271.1kWh, import 90.5kWh (thus net consumption (I+G-E) 141.2kWh); gas ~16m^3 (so 180kWh @ 11.1kWh/m^3).
Just passed 7MWh total PV generation, and also 3MWh on the export meter (ie over ~12 months) earlier this month.
Given last year's 6kWh/d + 3.8kWh/HDD12 gas consumption and 5.3kWh/d electricity, then for this winter I think that we should aim for 5.0--5.5kWh/d+3kWh/HDD12 gas or less and 4.5--5.0kWh/d electricity or less. (Or in other terms, approx 0.95+0.57/HDD12+1.94 = 2.98kgCO2/d+0.57kgCO2/HDD12, or approx 1.01tCO2/y for all but space heating and ~0.61tCO2/y space heating, for a total of ~1.6tCO2, vs ~1.7tCO2 avoided from PV exports to the grid.) These numbers should help see if we are on track even during rapidly-changing demand during the colder months, and would be close to zero-carbon. We have just missed the electricity target but met the gas target (207kWh) this month.
PV performance was disappointing but apparently September was a pretty average month including for sunshine.
HDD12=19 vs 7 last September.
2010/10 135 (4.4/d) -176 (-5.7/d) 241 (7.8/d) 20100930-17:18-GMT to 20101031-17:22-GMT (31d) Meter readings: electricity generation 175.7kWh, export 141.8kWh, import 101.5kWh (thus net consumption (I+G-E) 135.4kWh); gas ~22m^3 (so 241kWh @ 11.1kWh/m^3).
Just passed 7MWh total PV generation, and also 3MWh on the export meter (ie over ~12 months) earlier this month.
Consumption this month well within target 5.0--5.5kWh/d+3kWh/HDD12 gas or less (limit 344kWh+ given HDD) and 4.5--5.0kWh/d electricity or less. (We met the limits each week except the first full week, for gas; year so far 4098kWh gas vs 4600 last year, 1375kWh electricity vs 1612 last year.)
Turned heating on mid-month.
HDD12=62 vs 32 last October.
2010/11 163 (5.4/d) -84 (-2.8/d) 654 (22/d) 20101031-17:22-GMT to 20101130-18:22-GMT (30d) Meter readings: electricity generation 83.5kWh, export 57.3kWh, import 136.6kWh (thus net consumption (I+G-E) 162.8kWh); gas ~59m^3 (so 654kWh @ 11.1kWh/m^3).
Gas consumption this month well under target ceiling of 5.0--5.5kWh/d+3kWh/HDD12 gas (limit 706kWh given HDD; actual demand 5kWh/d + 2.7kWh/HDD12) but missed target of 4.5--5.0kWh/d electricity or less. I think that the excess electricity over target is for the central heating pump.
SheevaPlug stayed off-grid all month though the SLA SoC indicator has been down to amber a couple of times.
HDD12=185 vs 78 last November. (Gas consumption was slightly up on last year for more than double the heat demand. Note that the year-to-date HDD12/demand of 1141 already exceeds the entire 2008 (1061) and 2009 (1068) years' totals! Gas consumption Jan--Nov 2010 is 4753kWh, and adding base use of 5kWh/d for December would only have reached 4908kWh, which would have been a ~20% saving over 2009.)
2010/12 181 (5.9/d) -25 (-0.81/d) 1031 (33/d) 20101130-18:22-GMT to 20101231-18:06-GMT (31d) Meter readings: electricity generation 25.0kWh, export 12.1kWh, import 168.5kWh (thus net consumption (I+G-E) 181.4kWh); gas ~93m^3 (so 1031kWh @ 11.1kWh/m^3).
Gas consumption this month well target ceiling of 5.0--5.5kWh/d+3kWh/HDD12 gas (limit 706kWh given HDD; actual demand 5kWh/d + 2.7kWh/HDD12) but missed target of 4.5--5.0kWh/d electricity or less; the electricity target probably needs an HDD12 component, eg +0.1HDD12.
SheevaPlug was on-grid (on mains electricity) for a total of ~20 days due to very poor insolation (less than half that expected, though the UK as a whole was generally drier and sunnier than normal) and was on-grid at the turn of the year.
HDD12=340 vs 250 last December (but gas consumption slightly down); easily the highest heat demand in any one month since we started conserving (the coldest December in 100 years), with only this January coming close (HDD12=316).
2010 total (Jan--Dec) 1554 (4.3/d) -3547 (-9.7/d) 5784 (16/d) 20091231-18:07-GMT to 20101231-18:06-GMT (365d) Electricity consumption down ~20% on last year, generation up ~20%; now ~130%/2MWh net exporters. Actual metered imports 1051.5kWh. Consumption roughly bounded by 4.5kWh/d+0.2kWh/HDD12.
Gas consumption down ~7% on last year though very cold (2010 was ranked the 12th coldest year in the previous one hundred years).
HDD12 for the year 1481 (vs 1068 for 2009) thus heating efficiency up ~50% from 2009 and maybe doubled since 2008, at ~5kWh/d+2.7kWh/HDD12 cf ~8kWh/d+5.7kWh/HDD12 in 2008.
kgCO2 668kg (1.8/d) -1525kg (-4.2/d) 1099kg (3.0/d) ~0.24tCO2 (~0.66kgCO2/d) gas/electricity emissions total for 4 people, based on 0.43kgCO2/kWh for electricity (netted) and 0.19kgCO2/kWh for gas.
Using the DEFRA emissions figures of gas@0.206kgCO2/kWh and electricity@0.523kgCO2/kWh (5-year rolling average) that iMeasure does would bring us to +1192kgCO2 (gas) and -1042kgCO2 (netted electricity) or 0.15tCO2 for the year (37kgCO2/person/year).
Our house would probably meet the Zero Carbon Hub proposed 11kgCO2e/m^2/y zero carbon homes standard.
Note that we haven't flown this year again, we still don't own a car and travel almost entirely by (usually-electrified) public transport.
2011 Daily PV Output


2011 data (kWh: e used 1578, e gen -3998, g 3936) and discussion...

See the daily PV generation logs for 2011 as .csv and .png chart.

I aim to continue to meet or beat the targets set at the end of 2010:

  • Electricity: 4.5kWh/d+0.2kWh/HDD12
  • Gas: 5kWh/d+3kWh/HDD12
MonthConsumed kWh (/d)Generated -kWh (/d)Consumed kWh (/d)PeriodNotes
2011/01 159 (5.1/d) -60 (-1.9/d) 816 (26/d) 20101231-18:06-GMT to 20110131-18:23-GMT (31d) Meter readings: electricity generation 59.9kWh, export 38.1kWh, import 137.0kWh (thus net consumption (I+G-E) 158.8kWh); gas ~73.5m^3 (so 816kWh @ 11.1kWh/m^3).
SheevaPlug on grid (on mains electricity) about 8 days in total this month; new larger off-grid storage (~21d) should reduce on-grid time next winter!
(Some notes... SheevaPlug off-grid evening of 3rd Jan as SLA controller indicator has finally gone green after some sunshine today. SheevaPlug SLA battery failed ~1am 9th Jan even after some decent sunshine in prev day or two, so definitely seems ill. SheevaPlug battery upgraded to 400Ah (~2kWh usable), going back off-grid evening of 14th Jan. Controller showing green and voltage 12V night of 27th; cut off to red/LVD just before 09:00 on 28th (with LiFePO4 carrying system for a few minutes until plug put back on-grid), battery voltage measured as 12.3V at controller minutes later but LVD not reset for about 3h after some sunshine.)
Electricity/gas targets met for month (though missed one week in the month).
HDD12=215 vs 316 last January. (Revised down from 231.)
2011/02 132 (4.7/d) -96 (-3.4/d) 574 (20/d) 20110131-18:23-GMT to 20110228-18:30-GMT (28d) Meter readings: electricity generation 96.2kWh, export 70.7kWh, import 106.9kWh (thus net consumption (I+G-E) 132.4kWh); gas ~52m^3 (so 574kWh @ 11.1kWh/m^3).
SheevaPlug off-grid all month.
Electricity targets met for month, gas targets just missed.
HDD12=132 vs 232 last February. (Revised down from 140.)
2011/03 158 (5.1/d) -284 (-9.2/d) 518 (17/d) 20110228-18:30-GMT to 20110331-17:20-GMT (31d) Meter readings: electricity generation 284.0kWh, export 239.2kWh, import 112.8kWh (thus net consumption (I+G-E) 157.6kWh); gas ~47m^3 (so 518kWh @ 11.1kWh/m^3).
Central heating off for the last few days of the month.
Electricity and gas targets met for month.
HDD12=146 vs 165 last March. (Revised down from 152.)
2011/04 122 (4.1/d) -518 (-17/d) 181 (6.0/d) 20110331-17:20-GMT to 20110430-17:12-GMT (30d) Meter readings: electricity generation 518.0kWh, export 465.8kWh, import 70.1kWh (thus net consumption (I+G-E) 122.3kWh); gas ~16m^3 (so 181kWh @ 11.1kWh/m^3).
Moved ADSL modem/WiFi off-grid early this month, saving maybe 0.2kWh/d mains.
Total generation from the initial rounds of PV passed 7MWh and on the newest round passed 1MWh this month.
Electricity and gas targets met for month.
HDD12=33 vs 82 last April. (Revised down from 36.)
2011/05 124 (4.0/d) -648 (-21/d) 176 (5.7/d) 20110430-17:12-GMT to 20110531-17:26-GMT (31d) Meter readings: electricity generation 647.7kWh, export 585.2kWh, import 61.1kWh (thus net consumption (I+G-E) 123.6kWh); gas ~16m^3 (so 176kWh @ 11.1kWh/m^3).
Moved ADSL modem/WiFi back on-grid mid month since main battery getting low.
Passed 9MWh total generation at the end of the month.
Electricity and gas targets met for month.
HDD12=24 vs 66 last May. (Revised down from 30.)
2011/06 120 (4.0/d) -591 (-20/d) 166 (5.5/d) 20110531-17:26-GMT to 20110630-17:08-GMT (30d) Meter readings: electricity generation 590.5kWh, export 531.7kWh, import 60.9kWh (thus net consumption (I+G-E) 119.7kWh); gas ~15m^3 (so 166kWh @ 11.1kWh/m^3).
Passed 8MWh generated on the first generation meter and 5MWh exported.
Electricity and gas targets met for month.
HDD12=10 vs 8 last June. (Revised down from 15.)
2011/07 122 (3.9/d) -587 (-19/d) 155 (5.0/d) 20110630-17:08-GMT to 20110731-17:15-GMT (31d) Meter readings: electricity generation 587.3kWh, export 527.5kWh, import 62.4kWh (thus net consumption (I+G-E) 122.2kWh); gas ~14m^3 (so 155kWh @ 11.1kWh/m^3).
SunnyBeam showing 9999.10kWh generated up to night of 2011/07/30; end of month generation readings sum to 10189.3kWh with 10022.3kWh on SunnyBeam. Carbon footprint so far for 2011 = approx -440kgCO2 ie negative; from ~490kgCO2 (gas) and -930kgCO2 (electricity).
Electricity and gas targets met for month.
HDD12=1 vs 0 last July.
2011/08 103 (3.0/d) -525 (-15/d) 105 (3.1/d) 20110731-17:15-GMT to 20110903-17:12-GMT (34d) Meter readings: electricity generation 524.8kWh, export 480.1kWh, import 58.3kWh (thus net consumption (I+G-E) 103.0kWh); gas ~9m^3 (so 105kWh @ 11.1kWh/m^3).
Away on holiday thus EOM readings delayed and consumption low.
Metered exports passed 6MWh and generation on the first meter passed 9MWh.
Electricity and gas targets met for month.
HDD12=2 (for just August) vs 4 last August. (First 3 days of Sept add little.)
2011/09 123 (4.6/d) -313 (-12/d) 163 (6.0/d) 20110903-17:12-GMT to 20110930-17:15-GMT (27d) Meter readings: electricity generation 313.0kWh, export 267.4kWh, import 77.8kWh (thus net consumption (I+G-E) 123.4kWh); gas ~15m^3 (so 163kWh @ 11.1kWh/m^3).
Electricity target met for month, gas not.
HDD12=6 (for all September) vs 19 last September.
2011/10 134 (4.3/d) -223 (-7.2/d) 170 (5.5d) 20110930-17:15-GMT to 20111031-18:28-GMT (31d) Meter readings: electricity generation 222.8kWh, export 186.9kWh, import 98.2kWh (thus net consumption (I+G-E) 134.1kWh); gas ~15m^3 (so 170kWh @ 11.1kWh/m^3).
Heating not turned on this month.
Electricity and gas targets met for month.
HDD12=31 vs 62 last October.
2011/11 149 (5.0/d) -87 (-2.9/d) 347 (12d) 20111031-18:28-GMT to 20111130-18:04-GMT (30d) Meter readings: electricity generation 86.6kWh, export 61.4kWh, import 122.9kWh (thus net consumption (I+G-E) 148.9kWh); gas ~31m^3 (so 347kWh @ 11.1kWh/m^3).
Heating turned on early this month. (The first time, briefly, on 4th Nov.)
Electricity and gas targets just missed for month.
HDD12=66 (second warmest UK November and autumn on record) vs 185 last November (and 78 the previous November).
2011/12 136 (4.4/d) -57 (-1.8/d) 566 (18/d) 20111130-18:04-GMT to 20111231-18:12-GMT (31d) Meter readings: electricity generation 57.3kWh, export 36.5kWh, import 116.6kWh (thus net consumption (I+G-E) 136.4kWh); gas ~51m^3 (so 566kWh @ 11.1kWh/m^3).
Slightly off month targets with house empty for 3 days over holidays but two parties here one weekend...
SheevaPlug still off-grid with plenty of juice in the main battery bank.
Electricity and gas targets met for month.
HDD12=154 vs 340 last December (and 250 the previous December).
2011 total (Jan--Dec) 1578 (4.32/d) -3988 (-10.9/d) 3936 (10.8/d) 20101231-18:06-GMT to 20111231-18:12-GMT (365d) Electricity import (1081kWh) and consumption close to 2010, generation up 10%. Electricity bill reports import for 2011 was 1093kWh with predicted costs for 2012 at current rates £192 (@~13p/kWh+sc). Gas consumption down more than 30% on last year with HDD12 down ~45% (to 820); Space heating demand has dropped to ~2.6kWh/HDD12 (from 2.7 last year). DHW has accounted for approximately 50% of gas/heat demand this year, so solar thermal to as much as halve that is looking increasingly attractive. Gas bill reports consumption for 2011 was 4168kWh with predicted costs for 2012 at current rates £286 (@~4p/kWh+sc).
kgCO2 678kg (1.9/d) -1715kg (-4.7/d) 748kg (2.0/d) ~-0.29tCO2 (~-0.79kgCO2/d) gas/electricity emissions total for 4 people, based on 0.43kgCO2/kWh for electricity (netted) and 0.19kgCO2/kWh for gas, ie slightly carbon negative.
Note that we haven't flown this year again, we still don't own a car and travel almost entirely by (usually-electrified) public transport.

In August I set up an account with The European Citizens Climate Cup to see how it compares with iMeasure. After a few minutes' data entry (and fighting some glitches such as the site not understanding 'net metering' ie meters that run backwards), I was able to look at some pretty diagrams including the following reinforcing my belief that we use less than half the typical UK electricity and gas for our location, etc:

EU Climate Cup CO2 Diagram

Also in August I reviewed the AlertMe energy-monitoring kit for The Register, which amongst other things has decided that our "always on" electricity load / baseload is ~70W, mainly the fridge I suspect.

As of iMeasure's upgrade in September, with a HDD12 base, iMeasure reports:

Baseload = 27.82 Slope = 2.89 R = 0.942
performance line heating chart

The near-1.0 R/correlation figure implies that we are efficient, using heating based on actual external temperature. If our DHW and cooking were separate this might be even closer to 1.

The baseload seems to be a weekly kWh figure, ie ~4kWh/d for DHW/cooking, which is about right.

The slope of 2.89, presumably kWh/HDD, is again fairly close to the 3kWh/HDD measured and used as a target. Possibly I should lower our target a little, especially as the data used goes back to 2009 and we've been improving! Note that the (high-heat-demand) points at the right-hand end are all from around the exceptionally cold start and end of 2010.

(The following day, possibly as a result of on-going bug-fixing, the numbers were shown as Baseload = 31.38 Slope = 3.01, so slightly higher but still including values from 2009 of course.)

Having gone back and entered data for previous meters I now have a 4-year energy chart from autumn 2007 to end-summer 2011.

  • Heating demand has dropped each year even though winters have not been getting milder.
  • Peak heating demand has dropped from ~63kWh/d in 2007/12 to ~38kWh/d in 2011/01.
  • The electricity meter from 2008/02 to 2009/09 was unratcheted and imeasure objects to readings going backwards so that part of the chart is dubious (subtracting our next export from PV generation for the period gives ~3.1MWh consumption for that period, not including what was used locally).
  • The extra historical data has adjusted our performance-line heating numbers to Baseload = 52.33 Slope = 3.17 R = 0.810, ie the old data makes us look worse now!
  • I discovered that I have lost all meter readings for 2009/07: annoying!
4-year energy consumption chart (kWh/week)

An implication of this, with 3kWh/HDD12 and a typical 1070HDD12 year (3.21MWh), and the fact that our current gas combi is actually only ~80% efficient, is that our true heat demand for CH is now ~2.5MWh/y (and falling). That's somewhat over the heat storage capacity of one mythical buried milk-tanker (20kl/20t) at a delta-T of 80K (~90kWh/t for a delta-T of 80K, ~1.8MWh/tanker). A couple of tonnes (~60kWh/t at a delta-T of 50K, thus ~120kWh) could carry enough from mid-summer into autumn to postpone using gas for DHW for an extra month or so.

Note that the SheevaPlug stayed off-grid all year after the battery upgrade (and some more tweaking of software to use less energy in the darkest periods) and I haven't seen the controller light on amber or red even in December.

Carbon footprint for heat'n'light this year indicates, mainly due to the milder weather, that we're slightly carbon-negative (circa -300kgCO2 for the house for 2011), which is good.

That's assuming 0.43kgCO2/kWh electricity for both consumption and generation: at 0.5kgCO2/kWh we're at about -500kg for 2011 by the looks of things (we're currently a net exporter until I can get ASHP in).

Definitely not Passivhaus, but with heating at ~50kWh/m^2/year (ie gas use at <4MWh/y cf national average of 18MWh/y) I don't think we're doing too badly.

Disappointingly our gross electricity consumption has gone up a tiny bit.

(Note that we no longer have any incandescent bulbs other than in the oven and (oddly) fridge---we're all CFL and LED energy-saving lamps---nor any other easy low-hanging fruit in this area, and indeed we may have to use a bit extra to dehumidify the house in winter.)

HDD12 for year 820, much lower than previous years, with gas consumption ~3936kWh, so if baseline (DHW+cooking) is still 5kWh/day that leaves 2111kWh for space heating, or a little under 2.6kWh/HDD12, so that can guide next year's target (down from 3.0kWh/HDD12).

As of 2011/12/31 is claiming Baseload = 50.46 Slope = 3.17 R = 0.810 under Weather Analysis.

Electricity targets could be tightened a little too, maybe more like 4kWh/d+0.15kWh/HDD12 (with mean consumption currently ~4.3kWh/d over the year).

2012 Daily PV Output


2012 data (kWh: e used 1542, e gen -3778, g 3707) and discussion...

See the daily PV generation logs for 2012 as .csv and .png chart.

The daily/monthly target for 2012 is to match 2011 stats, allowing some noise:

  • Electricity: 4kWh/d+0.15kWh/HDD12
  • Gas: 5kWh/d+2.6kWh/HDD12

MonthConsumed kWh (/d)Generated -kWh (/d)Consumed kWh (/d)PeriodNotes
2012/01 174 (5.6/d) -75 (-2.4/d) 669 (22/d) 20111231-18:12-GMT to 20120131-18:42-GMT (31d) Meter readings: electricity generation 75.3kWh, export 47.1kWh, import 145.6kWh (thus net consumption (I+G-E) 158.8kWh); gas ~60.3m^3 (so 669kWh @ 11.1kWh/m^3).
Electricity target missed for month with dehumifier on a lot to try to keep below 70%RH in the kitchen especially when laundry drying. Gas target missed (592kWh@169HDD for central heating) also, at ~3.1kWh/HDD, possibly also because of different attempts to keep humidity in check.
HDD12=169 vs 215 last January. (Compare with January 2008: HDD12=140, gas 1418kWh, electricity 214kWh.)
2012/02 153 (5.3/d) -148 (-5.1/d) 653 (23/d) 20120131-18:42-GMT to 20120229-18:11-GMT (29d) Meter readings: electricity generation 148.2kWh, export 113.3kWh, import 117.7kWh (thus net consumption (I+G-E) 152.6kWh); gas ~58.8m^3 (so 653kWh @ 11.1kWh/m^3).
Electricity (t:153kWh) missed and gas (t:549kWh CH) hit target for month. Was still using dehumidifier a little to control RH.
HDD12=211 vs 132 last February.
2012/03 139 (4.5/d) -371 (-12/d) 328 (11/d) 20120229-18:11-GMT to 20120331-17:18-GMT (31d) Meter readings: electricity generation 370.6kWh, export 370.6kWh, import 96.1kWh (thus net consumption (I+G-E) 139.3kWh); gas ~29.5m^3 (so 238kWh @ 11.1kWh/m^3).
Electricity and gas targets met for month.
From second week had MHRV off and windows open selectively, moved ADSL off-grid. Near end of month passed 10MWh on first generation meter, and turned central heating off.
HDD12=109 vs 146 last March; this one was third warmest on record.
2012/04 117 (3.9/d) -409 (-14/d) 147 (4.9/d) 20120331-17:18-GMT to 20120430-17:01-GMT (30d) Meter readings: electricity generation 408.9kWh, export 362.9kWh, import 70.9kWh (thus net consumption (I+G-E) 116.9kWh); gas ~13.3m^3 (so 147kWh @ 11.1kWh/m^3).
Electricity and gas targets met for month. I believe that the MHRV has probably helped with gas/heating in particular by helping us trap heat when available.
Passed 12MWh generation total according to (slightly conservative) SunnyBeam.
HDD12=108 vs 33 last April and 82 in April 2010; this April was wettest for over 100 years and very gloomy at times.
2012/05 128 (4.1/d) -524 (-17/d) 174 (5.6/d) 20120430-17:01-GMT to 20120531-21:15-GMT (31d) Meter readings: electricity generation 524.0kWh, export 460.0kWh, import 64.0kWh (thus net consumption (I+G-E) 128.0kWh); gas ~15.7m^3 (so 174kWh @ 11.1kWh/m^3).
Electricity and gas targets met for month.
Passed 8MWh exports.
HDD12=42 vs 24 last May and 66 in May 2010.
2012/06 103 (3.4/d) -507 (-17/d) 148 (4.9/d) 20120531-21:15-GMT to 20120630-18:23-GMT (30d) Meter readings: electricity generation 507.6kWh, export 443.6kWh, import 39.3kWh (thus net consumption (I+G-E) 103.2kWh); gas ~13.3m^3 (so 148kWh @ 11.1kWh/m^3).
Electricity and gas targets met for month.
Experimentally we are running appliances in the day to better match generation, although it probably does not minimise carbon footprint (vs the wee hours), and imports this month were ~39kWh compared to ~61kWh last June, so in the summer months there are some real options with load shifting.
Generation probably low in part because April to June this year has been the wettest second quarter in the UK since records began in 1910 and June was the second dullest recorded.
HDD12=7 vs 10 last June and 8 in June 2010.
2012/07 99 (3.2/d) -534 (-17/d) 126 (4.1/d) 20120630-18:23-GMT to 20120731-17:12-GMT (31d) Meter readings: electricity generation 534.4kWh, export 470.9kWh, import 35.3kWh (thus net consumption (I+G-E) 98.8kWh); gas ~11.4m^3 (so 125kWh @ 11.1kWh/m^3).
Electricity and gas targets met for month.
We continued running appliances/loads to match PV generation, even though the generally poor sunshine and unpredictable showers/cloud made that hard (oh for smart appliances / HAN / smart meters!).
We were away on holiday the last week of July, thus low consumption figures.
HDD12=1 vs 1 last July and 0 in July 2010.
2012/08 93 (3.0/d) -514 (-17/d) 101 (3.3/d) 20120731-17:12-GMT to 20120831-17:40-GMT (31d) Meter readings: electricity generation 513.5kWh, export 457.5kWh, import 37.2kWh (thus net consumption (I+G-E) 93.2kWh); gas ~9.1m^3 (so 101kWh @ 11.1kWh/m^3).
Electricity and gas targets met for month.
We continued running appliances/loads to match PV generation; next month we switch back to make the effect on imports clear in the monthly stats.
Most of the family was still away on holiday at the start of this month, and there other short breaks from home, thus low consumption figures.
This was probably the second-wettest summer on record.
It was chilly enough on the night of the 30th for condensation on the insides of the kids' (old 2G) windows at ~8°C.
HDD12=2 vs 2 last August and 4 in August 2010.
2012/09 126 (4.2/d) -391 (-13/d) 165 (3.3/d) 20120831-17:40-GMT to 20120930-16:55-GMT (30d) Meter readings: electricity generation 391.3kWh, export 341.8kWh, import 76.4kWh (thus net consumption (I+G-E) 125.9kWh); gas ~14.8m^3 (so 165kWh @ 11.1kWh/m^3).
Electricity target just missed, gas target met for month.
This month we switched back to running washing machine and dishwasher at night to minimise carbon footprint rather than our bills (first dishwasher run in early hours of 1st Sept, though ran dishwasher maintenance hot wash during gloriously-sunny 9th) to make the effect on imports (doubling!) clear in the monthly stats.
We had some dew outside our new triple glazing, which shows it working.
HDD12=17 vs 6 last September and 19 in September 2010.
2012/10 119 (3.8/d) -164 (-5.2/d) 152 (4.9/d) 20120930-16:55-GMT to 20121031-18:00-GMT (31d) Meter readings: electricity generation 164.1kWh, export 135.3kWh, import 90.3kWh (thus net consumption (I+G-E) 119.1kWh); gas ~13.7m^3 (so 152kWh @ 11.1kWh/m^3).
Electricity and gas targets met for month.
ADSL/broadband modem went back on-grid 5th Oct when off-grid battery getting low and not much sunshine was forecast.
Energy bills before Big 6 rises announced predicted cost for next 12 months at current rates of ~£190 for electricity (1.0MWh) and ~£250 for gas (3.4MWh).
Central heating not on yet! Dehumidifier run from time to time when RH above 70% (on ~4 days).
Was in DECC smart heating workshop!
HDD12=55 vs 31 last October and 62 in October 2010.
2012/11 149 (5.0/d) -90 (-3.0/d) 366 (12/d) 20121031-18:00-GMT to 20121130-18:03-GMT (30d) Meter readings: electricity generation 90.0kWh, export 61.6kWh, import 120.7kWh (thus net consumption (I+G-E) 149.1kWh); gas ~33.0m^3 (so 366kWh @ 11.1kWh/m^3).
Electricity and gas targets met for month.
Heating on 2012/11/06 18:15 after about 6 HDD12 each day since start of month. Total gas consumption about same as last November though much colder this.
One day no electricity at all was exported to the grid (and we imported 5.3kWh).
Kitchen MHRV installed 2012/11/26 (and test electronic TRV smart heating 2012/12/15 in one bedroom); significant dehumidifier use until then, including ~10l water removed in one day!
HDD12=133 vs 66 (347kWh gas use) last November and 185 (654kWh gas use) in November 2010.
2012/12 142 (4.6/d) -50 (-1.6/d) 677 (22/d) 20121130-18:03-GMT to 20121231-18:41-GMT (31d) Meter readings: electricity generation 49.9kWh, export 29.5kWh, import 121.8kWh (thus net consumption (I+G-E) 142.2kWh); gas ~61.0m^3 (so 677kWh @ 11.1kWh/m^3).
Electricity target met, gas target missed (by ~4%). (I think that we ran over in part because I spent far too much of three weeks in bed with colds/flu/whatever, with the heating up; and also because I stayed home at no point was the place entirely empty.)
Failed CRT TV replaced with lower-power larger-screen LED-backlit LCD model.
On 4 days no significant electricity was exported to the grid. (The off-grid system was straining in the gloom but still coping.)
A detailed analysis of energy by day shows, for example: a max gas consumption of 40kWh on one day, an apparent baseline of 9.2kWh/d and slope of 2.1kW/HDD12 for gas, and an almost complete lack of correlation between electricity and HDD. I also compute what electricity imports would be if gas heating were replaced with various CoPs of heat-pump, ignoring cooking.
HDD12=190 vs 154 last December and 328 for 2010; a fairly mild winter so far.
2012 total (Jan--Dec) 1542 (4.21/d) -3778 (-10.3/d) 3707 (10.1/d) 20111231-18:12-GMT to 20121231-18:41-GMT (366d) Electricity import (1015kWh) marginally down from 2011 (1081kWh) with consumption (1542kWh) back close to 2010 (1554kWh). Average 176W gross. Generation (3778kWh) sightly down to very close to PVGIS predictions (from 3988kWh). Gas consumption down about 6% (to 3707kWh, vs 3936kWh in 2011) even though HDD12 up ~25% (1043, vs 820 in 2011), ie the year was colder. Very similar final carbon-negative position to last year. (The utility bills came in dated 2013/01/02 stating gas consumption of 3474kWh over the last 12 months and electricity imports of 998kWh, from agent reads, so my figures are slightly conservative but close, with predicted costs at same rates for next 12 months £238 and £202 respectively.)
kgCO2 663kg (1.8/d) -1625kg (-4.4/d) 710kg (2.0/d) ~-0.26tCO2 (~-0.70kgCO2/d) gas/electricity emissions total for 4 people, based on 0.43kgCO2/kWh for electricity (netted) and 0.19kgCO2/kWh for gas, ie slightly carbon negative.
Note that we haven't flown this year again, we still don't own a car and travel almost entirely by (usually-electrified) public transport.

See five winters' worth of improving energy efficiency, noting that winter 2011/2012 was very mild. A crude analysis of exported imeasure data as at 2012/08/08 indicates that computed baseline (kWh/d) and slope ((kWh/d)/(HDD12/d)) for each year have been (roughly in line with previous estimates):

YearBaseline (kWh/d)Slope ((kWh/d)/(HDD12/d))year kWhyear HDD12

We became a 'SuperHome' (number 119) early in 2012, with nominal carbon savings (ignoring our PV) of 62%.

2013 Daily PV Output


2013 data (kWh: e used 1658, e gen -3740, g 3859) and discussion...

See the daily PV generation logs for 2013 as .csv and .png chart.

MonthConsumed kWh (/d)Generated -kWh (/d)Consumed kWh (/d)PeriodNotes
2013/01 155 (5.0/d) -55 (-1.8/d) 754 (24/d) 20121231-18:41-GMT to 20130131-23:23-GMT (31d) Meter readings: electricity generation 54.6kWh, export 35.1kWh, import 135.2kWh (thus net consumption (I+G-E) 154.5kWh); gas ~67.9m^3 (so 754kWh @ 11.1kWh/m^3).
Exports passed 10MWh. Several days with no generation/exports due to snow.
HDD12=231 vs 169 last January.
2013/02 124 (4.4/d) -119 (-4.2/d) 792 (28/d) 20130131-23:23-GMT to 20130228-18:27-GMT (28d) Meter readings: electricity generation 118.8kWh, export 91.8kWh, import 98.7kWh (thus net consumption (I+G-E) 123.7kWh); gas ~71.4m^3 (so 792kWh @ 11.1kWh/m^3).
Generation passed 15MWh according to SunnyBeam.
HDD12=230 vs 211 last February.
2013/03 147 (4.7/d) -213 (-6.9/d) 721 (23/d) 20130228-18:27-GMT to 20130331-17:15-GMT (31d) Meter readings: electricity generation 213.3kWh, export 167.2kWh, import 100.9kWh (thus net consumption (I+G-E) 147.0kWh); gas ~74.9m^3 (so 721kWh @ 11.1kWh/m^3).
Imports passed 4MWh since import meter fitted (2009/09/21).
HDD12=254 vs 109 last March: coldest month here since 2010/12, second coldest UK March on record.
2013/04 127 (4.2/d) -454 (-15/d) 330 (11/d) 20130331-17:15-GMT to 20130430-17:25-GMT (30d) Meter readings: electricity generation 454.3kWh, export 400.2kWh, import 72.8kWh (thus net consumption (I+G-E) 126.9kWh); gas ~29.7m^3 (so 330kWh @ 11.1kWh/m^3).
Central heating off this month after prolonged cold winter. MHRV on mainly at night until the end of the month.
Gas cooker/stove removed at the end of the month; switched to electric oven and induction hob which should be about carbon neutral for now but reduce footprint as the GB grid gets greener and is a step closer to eliminating direct fossil fuel use in the house.
HDD12=121 vs 108 last April.
2013/05 122 (3.9/d) -524 (-17/d) 138 (4.5/d) 20130430-17:25-GMT to 20130531-17:20-GMT (31d) Meter readings: electricity generation 524.2kWh, export 454.3kWh, import 52.1kWh (thus net consumption (I+G-E) 122.0kWh); gas ~12.4m^3 (so 138kWh @ 11.1kWh/m^3). Passed 11MWh on export meter.
MHRV on occasionally, mainly at night, while exterior temperatures low.
With switch of cooking from gas to electricity I am expecting gas usage to drop ~1kWh/d and electricity usage to rise much less than that (preferably well under 0.5kWh/d to reduce overall carbon footprint). This month both fuels at record non-holiday lows.
HDD12=50 vs 42 last May, ending the coldest spring in 50 years.
2013/06 143 (4.8/d) -554 (-18/d) 136 (4.5/d) 20130531-17:20-GMT to 20130630-17:10-GMT (30d) Meter readings: electricity generation 554.3kWh, export 474.7kWh, import 63.9kWh (thus net consumption (I+G-E) 143.5kWh); gas ~12.3m^3 (so 136kWh @ 11.1kWh/m^3).
Some extra grid consumption over expected (~0.4kWh/d) because off-grid system down (restored at end of month), and Air Quality Egg in use (AQE continues to draw ~0.1kWh/d from grid).
Gas usage down ~0.5kWh as last month over previous year and maybe ~0.3kWh/d extra electricity usage attributable to switch from gas cooking; too early to be clear.
HDD12=8 vs 7 last June.
2013/07 106 (4.0/d) -601 (-23/d) 91 (3.4/d) 20130630-17:10-GMT to 20130727-07:35-GMT (26.5d) Meter readings: electricity generation 601.2kWh, export 536.2kWh, import 41.2kWh (thus net consumption (I+G-E) 106.2kWh); gas ~8.2m^3 (so 91kWh @ 11.1kWh/m^3).
Short month as went on holiday on 27th, though gas consumption and HDD good to month end anyway, eg zero gas consumption during first holiday week with everyone away. Electricity consumption ~1.4kWh/d during first holiday week: mainly fridge+ADSL, of which ~50% imported.
HDD12=0 vs 1 last July.
2013/08 106 (3.4/d) -620 (-17/d) 58 (1.6/d) 20130727-07:35-GMT to 20130831-17:01-GMT (35.5d) Meter readings: electricity generation 620.8kWh, export 568.2kWh, import 53.9kWh (thus net consumption (I+G-E) 105.2kWh); gas ~5.3m^3 (so 58kWh @ 11.1kWh/m^3).
Long month extending from morning of 27th July when we went on holday. No gas consumption at all 28th July to 9th Aug (I managed without DHW home alone for one week, and all cooking is now electric).
HDD12=0 vs 2 last August.
2013/09 151 (5.0/d) -309 (-10/d) 117 (3.9/d) 20130831-17:01-GMT to 20130930-17:03-GMT (30d) Meter readings: electricity generation 309.0kWh, export 256.0kWh, import 98.3kWh (thus net consumption (I+G-E) 151.3kWh); gas ~10.5m^3 (so 117kWh @ 11.1kWh/m^3).
HDD12=10 vs 17 last September.
2013/10 141 (4.5/d) -161 (-5.2/d) 102 (3.3/d) 20130930-17:03-GMT to 20131031-16:48-GMT (31d) Meter readings: electricity generation 160.7kWh, export 129.1kWh, import 109.0kWh (thus net consumption (I+G-E) 140.6kWh); gas ~9.2m^3 (so 102kWh @ 11.1kWh/m^3).
Passed 13MWh exports this month.
Heating not on yet, though OpenTRV units (mainly V0.09) in and running.
HDD12=23 vs 55 last October.
2013/11 167 (5.6/d) -89 (-3.0/d) 301 (10/d) 20131031-16:48-GMT to 20131131-18:15-GMT (30d) Meter readings: electricity generation 88.7kWh, export 58.5kWh, import 136.6kWh (thus net consumption (I+G-E) 166.8kWh); gas ~27.1m^3 (so 301kWh @ 11.1kWh/m^3).
2013/11/11: two 3G units' outer panes shattered when outside temps reached ~3C overnight!
2013/11/15: heating on (for a couple of days it had been on intermittently), under OpenTRV control.
HDD12=139 vs 133 last November (@~3kWh/d base DHW use, implies ~1.5kWh/HDD12 for CH).
2013/12 170 (5.5/d) -50 (-1.6/d) 418 (13/d) 20131131-18:15-GMT to 20131231-19:25-GMT (31d) Meter readings: electricity generation 49.8kWh, export 27.5kWh, import 147.3kWh (thus net consumption (I+G-E) 169.6kWh); gas ~37.6m^3 (so 418kWh @ 11.1kWh/m^3).
2013/12/27: house down to ~12°C (minimum ~10°C) after three days empty and only frost protection with outside temps down to ~3°C.
All rooms now OpenTRV heating zones.
HDD12=152 vs 190 last December and 154 previously (@~3kWh/d base DHW use, implies ~2.1kWh/HDD12 for CH, ~20--40% heating efficiency improvement since 2011 allowing for 1kWh/d cooking reduction).
2013 total (Jan--Dec) 1659 (4.54/d) -3750 (-10.3/d) 3859 (10.6/d) 20121231-18:41-GMT to 20131231-19:25-GMT (365d) Electricity import (1108kWh vs 1015kWh) and gross consumption (1659kWh vs 1542kWh) marginally up from 2012 as expected due to switch to electric cooker. Generation essentially unchanged. Average 189W gross. Gas consumption up a little (3859kWh vs 3707kWh) given ~150kWh cooking elimination but HDD12 (1218 vs 1043) suggests improved heating efficiency (at ~2.1kWh/HDD12). (The kWh figures agree closely with end-of year bills which also, wrongly alas, project total bills for next year for all energy of just over £400!) Similar final carbon-negative position to last year.
kgCO2 713kg (2.0/d) -1612kg (-4.4/d) 733kg (2.0/d) ~-0.17tCO2 (~-0.45kgCO2/d) gas/electricity emissions total for 4 people, based on 0.43kgCO2/kWh for electricity (netted) and 0.19kgCO2/kWh for gas, ie slightly carbon negative.
Note that we haven't flown this year again, we still don't own a car and travel almost entirely by (usually-electrified) public transport.

Another iMeasure graph suggests year-on-year heating efficiency improvements:

normalised gas consumption to 2013

with declining kWh used each year after allowing for weather differences (HDD), starting at 8.4MWh in 2008 and declining to 3.3MWh in 2013.

2014 Daily PV Output


2014 data (kWh: e used 1678, e gen -3944, g 2997)...

See the daily PV generation logs for 2014 as .csv and .png chart.

MonthConsumed kWh (/d)Generated -kWh (/d)Consumed kWh (/d)PeriodNotes
2014/01 190 (6.1/d) -76 (-2.4/d) 532 (17/d) 20131231-19:25-GMT to 20140131-19:02-GMT (31d) Meter readings: electricity generation 75.9kWh, export 48.5kWh, import 162.6kWh (thus net consumption (I+G-E) 190.0kWh); gas ~47.9m^3 (so 532kWh @ 11.1kWh/m^3).
Imports passed 5MWh. Very mild and wet this month.
HDD12=156 vs 231 last January (implying gas 2.8kWh/HDD12 at 3kWh/d baseload).
2014/02 131 (4.7/d) -145 (-5.2/d) 461 (16/d) 20140131-19:02-GMT to 20140228-18:17-GMT (28d) Meter readings: electricity generation 144.8kWh, export 117.5kWh, import 103.5kWh (thus net consumption (I+G-E) 130.8kWh); gas ~41.5m^3 (so 461kWh @ 11.1kWh/m^3).
Boiler/CH problems this month: internal bypass in ~10-year-old boiler apparently inadequate following (furred-up) primary heat-exchanger replacement mid-month, so had to leave one rad on permanently which wastes energy. External bypass fitted 2014/03/14.
Mildest February (lowest HDD12) in my records.
HDD12=127 vs 230 last February.
2014/03 162 (5.2/d) -341 (-11/d) 365 (12/d) 20140228-18:17-GMT to 20140331-18:08-GMT (31d) Meter readings: electricity generation 340.9kWh, export 286.3kWh, import 107.0kWh (thus net consumption (I+G-E) 161.6kWh); gas ~32.9m^3 (so 365kWh @ 11.1kWh/m^3).
HDD12=112 vs 254 last March.
2014/04 123 (4.1/d) -437 (-15/d) 137 (4.6/d) 20140331-18:08-GMT to 20140430-17:48-GMT (30d) Meter readings: electricity generation 436.7kWh, export 387.4kWh, import 73.6kWh (thus net consumption (I+G-E) 123.1kWh); gas ~12.3m^3 (so 137kWh @ 11.1kWh/m^3).
Passed 14MWh exports.
Heating officially off at the boiler 25th, though only operating intermittently before then.
HDD12=44 vs 121 last April.
2014/05 136 (4.4/d) -528 (-17/d) 132 (4.2/d) 20140430-17:48-GMT to 20140531-17:30-GMT (31d) Meter readings: electricity generation 527.5kWh, export 458.2kWh, import 66.6kWh (thus net consumption (I+G-E) 135.9kWh); gas ~11.9m^3 (so 132kWh @ 11.1kWh/m^3).
Passed 4MWh generation on the newest 1.29kWp string, and passed 20MWh generation for entire system according to the Sunny Beam.
HDD12=19 vs 50 last May.
2014/06 135 (4.5/d) -631 (-21/d) 116 (3.9/d) 20140531-17:30-GMT to 20140630-18:00-BST (30d) Meter readings: electricity generation 631.2kWh, export 552.7kWh, import 56.7kWh (thus net consumption (I+G-E) 135.2kWh); gas ~10.5m^3 (so 116kWh @ 11.1kWh/m^3).
Savings from automatically switching the ADSL modem off-grid ~62% of the month when the batteries were full amounts to perhaps (7.7W*24*30*.62) or ~3.4kWh, ie 2.5% of gross consumption.
HDD12=2 vs 8 last June.
2014 running total (Jan--Jun) 877 (4.8/d) -2157 (-12/d) 1742 (9.6/d) 20131231-19:25-GMT to 20140630-18:00-GMT (181d) Implies total footprint to date -227kgCO2: electricity (average 202W gross) -558kgCO2e @ 0.43kgCO2/kWh (netted), gas 330kgCO2e @ 0.19kgCO2/kWh.
2014/07 100 (4.1/d) -496 (-20/d) 85 (3.5/d) 20140630-18:00-BST to 20140725-06:38-GMT (24.5) Meter readings: electricity generation 496.3kWh, export 441.0kWh, import 44.9kWh (thus net consumption (I+G-E) 100.2kWh); gas ~7.7m^3 (so 85kWh @ 11.1kWh/m^3).
Short measurement month due to fortnight holiday starting in last week of July.
Import of ~9.5kWh vs gross consumption of 23kWh (1.7kWh/d) over the fortnight (well, 13.5d!) that we were away (with most things turned off other than the fridge/freezer (est ~60%/40W), security lighting / clocks / phones / etc, ADSL Internet modem (~10%/8W), and MHRV fans (~5%/4W) as ~70W baseload total vs ~200W 6M average), is consistent with the average daylight hours that the PV will have covered load. (1.8kWh of gas use was recorded between the last reading on the holiday morning and the first reading on return, which is likely a bit of DHW for washing!)
Passed 17MWh generation on the first generation meter/FiT (75% of the roof).
HDD12=0 for whole of calendar July vs 0 last July.
2014/08 114 (3.0/d) -650 (-17/d) 83 (2.2/d) 20140725-06:38-GMT to 20140831-17:07-GMT (37.5) Meter readings: electricity generation 650.1kWh, export 595.9kWh, import 60.0kWh (thus net consumption (I+G-E) 114.2kWh); gas ~7.5m^3 (so 83kWh @ 11.1kWh/m^3).
Long measurement month due to fortnight holiday starting in last week of July.
Passed 16MWh exported, of which nearly 600kWh this (extended) month alone.
HDD12=5 for whole of calendar August vs 0 last August.
2014/09 130 (4.3/d) -326 (-11/d) 99 (3.3/d) 20140831-17:07-GMT to 20140930-18:43-GMT (30) Meter readings: electricity generation 325.7kWh, export 276.5kWh, import 80.4kWh (thus net consumption (I+G-E) 129.6kWh); gas ~8.9m^3 (so 99kWh @ 11.1kWh/m^3).
HDD12=5 vs 10 last September.
2014/10 143 (4.6/d) -175 (-5.6/d) 121 (3.9/d) 20140930-18:43-GMT to 20141031-17:51-GMT (31) Meter readings: electricity generation 174.6kWh, export 141.4kWh, import 110.2kWh (thus net consumption (I+G-E) 143.4kWh); gas ~10.9m^3 (so 121kWh @ 11.1kWh/m^3).
Passed cumulative 18MWh generation on the first meter (3 strings).
HDD12=15 vs 23 last October.
2014/11 155 (5.2/d) -86 (-2.9/d) 282 (9.4/d) 20141031-17:51-GMT to 20141130-18:29-GMT (30) Meter readings: electricity generation 85.9kWh, export 61.6kWh, import 131.0kWh (thus net consumption (I+G-E) 155.3kWh); gas ~25.4m^3 (so 282kWh @ 11.1kWh/m^3).
Heating tested 6th, see graph and notes, and on properly after the 11th roughly.
Expecting daily electricity consumption in Nov/Dev/Jan to be very roughly: 1kWh each for cooking, washing m/c, dishwasher, fridge/freezer, boiler, with about 0.5kWh for other things mainly TV and lighting.
HDD12=79 vs 139 last November (ending the third warmest autumn on record).
2014/12 159 (5.1/d) -54 (-1.8/d) 585 (19/d) 20141130-18:29-GMT to 20141231-19:25-GMT (31) Meter readings: electricity generation 54.4kWh, export 34.8kWh, import 139.4kWh (thus net consumption (I+G-E) 159.0kWh); gas ~62.7m^3 (so 585kWh @ 11.1kWh/m^3).
Rolled 6MWh imports over New Year's night.
Christmas at home and lots of OpenTRV testing may have bumped up gas use!
HDD12=183 vs 152 last December (ending the warmest UK year on record)
2014 total (Jan--Dec) 1678 (4.60/d) -3944 (-10.8/d) 2997 (8.21/d) 20131231-19:25-GMT to 20141231-19:25-GMT (365d) Gas consumption an all-time low of just under 3MWh, probably because 2014 was a record warm year (HDD12 of 747 vs 1218 for 2013).
Ecotricity estimates our annual electricity imports at just under 1.1MWh (and our bill was a bit over £10 less than this time last year), and annual gas use at ~2.8MWh (and our bill was a bit over £8 more than this time last year), with an estimated total not much over £400 per year. Mean gross electricity consumption ~200W, import ~125W.
kgCO2 722kg (2.0/d) -1696kg (-4.6/d) 569kg (1.6/d) ~-0.41tCO2 (~-1.1kgCO2/d) gas/electricity emissions total for 4 people, based on 0.43kgCO2/kWh for electricity (netted) and 0.19kgCO2/kWh for gas, ie slightly carbon negative.
Note that we haven't flown this year again, we still don't own a car and travel almost entirely by (usually-electrified) public transport.
2015 Daily PV Output


2015 data (kWh: e used 1786, e gen -3229, g 2862)...

See the daily PV generation logs for 2015 as .csv and .png chart.

MonthConsumed kWh (/d)Generated -kWh (/d)Consumed kWh (/d) PeriodNotes
2015/01 181 (5.8/d) -75 (-2.4/d) 606 (20/d) 20141231-19:25-GMT to 20150131-19:50-GMT (31d) Meter readings: electricity generation 74.6kWh, export 49.4kWh, import 155.8kWh (thus net consumption (I+G-E) 181.0kWh); gas ~54.6m^3 (so 606kWh @ 11.1kWh/m^3).
Imports passed 6MWh right at the start of the month.
HDD12=202 vs 156 last January (implying gas 2.5kWh/HDD12 at 3kWh/d baseload, or 2.7kWh/HDD12 at 1.7kWh/d baseload that iMeasure currently estimates though iMeasure claims a slope of ~1kWh/d from last year's data).
2015/02 155 (5.5/d) -115 (-4.1/d) 525 (19/d) 20150131-19:50-GMT to 20150228-19:12-GMT (28d) Meter readings: electricity generation 115.1kWh, export 85.0kWh, import 125.3kWh (thus net consumption (I+G-E) 155.4kWh); gas ~47.3m^3 (so 525kWh @ 11.1kWh/m^3).
HDD12=199 vs 127 last February.
2015/03 161 (5.2/d) -294 (-9.5/d) 352 (11/d) 20150228-19:12-GMT to 20150331-17:40-GMT (31d) Meter readings: electricity generation 294.4kWh, export 242.1kWh, import 109.0kWh (thus net consumption (I+G-E) 161.3kWh); gas ~31.7m^3 (so 352kWh @ 11.1kWh/m^3).
All OpenTRV local heating programmes cancelled by end of month (though heating nominally still on centrally in case of cold snap).
HDD12=138 vs 112 last March.
2015/04 143 (4.5/d) -507 (-17/d) 132 (4.4/d) 20150331-17:40-GMT to 20150430-18:23-GMT (30d) Meter readings: electricity generation 506.7kWh, export 438.5kWh, import 65.8kWh (thus net consumption (I+G-E) 134.0kWh); gas ~11.9m^3 (so 132kWh @ 11.1kWh/m^3).
Passed 17MWh exports. Note that self-consumption was over 50%. Sunniest April in the UK on record.
HDD12=73 vs 44 last April and 121 the year before.
2015/05 148 (4.8/d) -583 (-19/d) 132 (4.3/d) 20150430-18:23-GMT to 20150561-18:20-GMT (31d) Meter readings: electricity generation 582.7kWh, export 502.5kWh, import 67.4kWh (thus net consumption (I+G-E) 147.6kWh); gas ~11.9m^3 (so 132kWh @ 11.1kWh/m^3).
Passed 24MWh generation (19MWh on first meter, 5MWh on second).
HDD12=30 vs 19 last May and 50 the year before.
2015/06 140 (4.7/d) -642 (-21/d) 112 (3.7/d) 20150561-18:20-GMT to 20150630-17:18-GMT (30d) Meter readings: electricity generation 642.2kWh, export 562.9kWh, import 60.7kWh (thus net consumption (I+G-E) 140.0kWh); gas ~10.1m^3 (so 112kWh @ 11.1kWh/m^3).
Passed 18MWh metered exports.
HDD12=7 vs 2 last June and 8 the year before.
2015 running total (Jan--Jun) 919 (5.1/d) -2216 (-12/d) 1860 (10/d) 20141231-19:25-GMT to 20150630-17:18-GMT (181d) Implies total footprint to date -205kgCO2: electricity (average 212W gross) -558kgCO2e @ 0.43kgCO2/kWh (netted), gas 353kgCO2e @ 0.19kgCO2/kWh.
2015/07 129 (4.2/d) -566 (-18/d) 98 (3.2/d) 20150630-17:18-GMT to 20150731-18:49-GMT (31d) Meter readings: electricity generation 566.8kWh, export 494.7kWh, import 56.6kWh (thus net consumption (I+G-E) 128.7kWh); gas ~8.8m^3 (so 98kWh @ 11.1kWh/m^3).
Passed 20MWh generation on the first meter, >25MWh total.
HDD12=1 vs 0 last July and 0 the year before.
2015/08 114 (3.6/d) -420 (-14/d) 85 (2.8/d) 20150731-18:49-GMT to 20150831-18:06-GMT (31d) Meter readings: electricity generation 419.6kWh, export 368.5kWh, import 62.8kWh (thus net consumption (I+G-E) 113.9kWh); gas ~7.7m^3 (so 85kWh @ 11.1kWh/m^3).
Passed 19MWh exports.
HDD12=0 vs 5 last August and 0 the year before.
2015/09 147 (4.9/d) -340 (-11/d) 113 (3.8/d) 20150831-18:06-GMT to 20150930-16:41-GMT (30d) Meter readings: electricity generation 339.8kWh, export 282.7kWh, import 89.9kWh (thus net consumption (I+G-E) 147.0kWh); gas ~10.2m^3 (so 113kWh @ 11.1kWh/m^3).
Passed 19MWh exports.
HDD12=14 vs 5 last September and 10 the year before.
2015/10 140 (4.5/d) -178 (-5.7/d) 123 (4.0/d) 20150930-16:41-GMT to 20151031-16:28-GMT (31d) Meter readings: electricity generation 177.8kWh, export 137.9kWh, import 100.5kWh (thus net consumption (I+G-E) 140.4kWh); gas ~11.1m^3 (so 123kWh @ 11.1kWh/m^3).
Started work on IWI on north wall upstairs; family away for 1 week.
HDD12=32 vs 15 last October and 23 the year before.
2015/11 168 (5.6/d) -62 (-2.1/d) 297 (9.9/d) 20151031-16:28-GMT to 20151130-17:12-GMT (30d) Meter readings: electricity generation 62.4kWh, export 37.0kWh, import 143.3kWh (thus net consumption (I+G-E) 168.7kWh); gas ~26.8m^3 (so 297kWh @ 11.1kWh/m^3).
Heating on (in part) from 1st Nov.
Continuing work on IWI on north wall upstairs.
HDD12=63 vs 79 last November and 139 the year before.
2015/12 168 (5.4/d) -47 (-1.5/d) 285 (9.2/d) 20151130-17:12-GMT to 20151231-19:20-GMT (31d) Meter readings: electricity generation 46.5kWh, export 25.1kWh, import 146.1kWh (thus net consumption (I+G-E) 167.5kWh); gas ~25.7m^3 (so 285kWh @ 11.1kWh/m^3).
Finished work on IWI on north wall upstairs.
HDD12=38 (warmest December by far in these records) vs 183 last December and 152 the year before.
2015 total (Jan--Dec) 1786 (4.89/d) -3829 (-10.5/d) 2862 (7.84/d) 20141231-19:25-GMT to 20151231-19:20-GMT (365d) Gas consumption an all-time low of under 3MWh and beating last year, with a very warm December though slightly higher HDD12 (797) than last year (747) against a more typical 1218 for 2013.
Not clear why electricity use up; possibly heavier use of dehumidifier in part.
Note that grid electricity carbon intensity dropped significantly in 2015, which would reduce the benefit of our electricity exports, but previous (0.43kgCO2/kWh) figures will be used for comparison.
kgCO2 768kg (2.1/d) -1646kg (-4.5/d) 554kg (1.5/d) ~-0.32tCO2 (~-0.9kgCO2/d) gas/electricity emissions total for 4 people, based on 0.43kgCO2/kWh for electricity (netted) and 0.19kgCO2/kWh for gas, ie slightly carbon negative.
Note that we haven't flown this year again, we still don't own a car and travel almost entirely by (usually-electrified) public transport.
2016 Daily PV Output


2016 data (kWh: e used 1831, e gen -3739, g 3851)...

See the daily PV generation logs for 2016 as .csv and .png chart.

MonthConsumed kWh (/d)Generated -kWh (/d)Consumed kWh (/d) PeriodNotes
2016/01 196 (6.3/d) -71 (-2.3/d) 593 (19/d) 20151231-19:20-GMT to 20160131-17:48-GMT (31d) Meter readings: electricity generation 71.0kWh, export 45.3kWh, import 169.9kWh (thus net consumption (I+G-E) 195.6kWh); gas ~53.4m^3 (so 593kWh @ 11.1kWh/m^3).
Generation on the first meter passed 21MWh.
HDD12=173 vs 202 last January.
2016/02 160 (5.5/d) -141 (-4.9/d) 561 (19/d) 20160131-17:48-GMT to 20160229-10:09-GMT (29d) Meter readings: electricity generation 104.7kWh, export 104.9kWh, import 124.4kWh (thus net consumption (I+G-E) 160.2kWh); gas ~50.5m^3 (so 561kWh @ 11.1kWh/m^3).
HDD12=172 vs 199 last February.
2016/03 180 (5.8/d) -274 (-8.8/d) 497 (16/d) 20160229-10:09-GMT to 20160331-17:35-GMT (31d) Meter readings: electricity generation 273.2kWh, export 220.7kWh, import 127.0kWh (thus net consumption (I+G-E) 179.5kWh); gas ~49.7m^3 (so 497kWh @ 11.1kWh/m^3).
Passed 20MWh exports.
HDD12=162 vs 138 last March.
2016/04 143 (4.8/d) -455 (-15/d) 192 (6.4/d) 20160331-17:35-GMT to 20160430-17:15-GMT (30d) Meter readings: electricity generation 455.0kWh, export 392.8kWh, import 80.4kWh (thus net consumption (I+G-E) 142.6kWh); gas ~17.3m^3 (so 192kWh @ 11.1kWh/m^3).
HDD12=104 vs 73 last April.
2016/05 147 (4.7/d) -756 (-19/d) 141 (4.6/d) 20160430-17:15-GMT to 20160531-17:41-GMT (31d) Meter readings: electricity generation 575.7kWh, export 497.5kWh, import 68.9kWh (thus net consumption (I+G-E) 147.1kWh); gas ~12.7m^3 (so 141kWh @ 11.1kWh/m^3).
Passed 21MWh exports, and 22MWh+6MWh generation on the two meters.
HDD12=23 vs 30 last May.
2016/06 132 (4.4/d) -474 (-16/d) 110 (3.7/d) 20160531-17:41-GMT to 20160630-16:27-GMT (30d) Meter readings: electricity generation 474.2kWh, export 404.4kWh, import 62.3kWh (thus net consumption (I+G-E) 132.1kWh); gas ~9.9m^3 (so 110kWh @ 11.1kWh/m^3).
Annual supplier statements state that we are using/importing ~3kWh/d electricity and ~7kWh/d gas on average for the year.
HDD12=2 vs 7 last June and 2 the previous June.
2016 running total (Jan--Jun) 957 (5.3/d) -1990 (-11/d) 2093 (12/d) 20151231-19:20-GMT to 20160630-16:27-GMT (182d) Implies total footprint to date -46kgCO2: electricity (average 219W gross) -444kgCO2e @ 0.43kgCO2/kWh (netted), gas 398kgCO2e @ 0.19kgCO2/kWh.
2016/07 135 (4.4/d) -585 (-19/d) 126 (4.0/d) 20160630-16:27-GMT to 20160731-17:45-GMT (31d) Meter readings: electricity generation 585.0kWh, export 510.6kWh, import 60.9kWh (thus net consumption (I+G-E) 135.3kWh); gas ~11.3m^3 (so 126kWh @ 11.1kWh/m^3).
HDD12=0 vs 1 last July and 0 the previous July.
2016/08 102 (3.0/d) -563 (-17/d) 75 (2.2/d) 20160731-17:45-GMT to 20160903-17:46-GMT (34d) Meter readings: electricity generation 563.0kWh, export 508.8kWh, import 48.1kWh (thus net consumption (I+G-E) 102.3kWh); gas ~6.8m^3 (so 75kWh @ 11.1kWh/m^3).
'Long' month since away on holiday a few days into September. Zero gas use and HDD while away. Electricity use ~1.3kWh/d while away, 50% covered by generation.
Latest Ecotricity projections for our annual imports: e ~1300kWh, g ~3300kWh.
HDD12=0 (calendar month) vs 0 last August and 5 the previous August.
2016/09 132 (4.9/d) -254 (-9.4/d) 103 (3.8/d) 20160903-17:46-GMT to 20160930-16:43-GMT (27d) Meter readings: electricity generation 254.4kWh, export 210.4kWh, import 87.9kWh (thus net consumption (I+G-E) 131.9kWh); gas ~9.3m^3 (so 103kWh @ 11.1kWh/m^3).
'Short' month since away on holiday a few days into September.
The load supposedly moved off-grid (~0.3kWh/d) does not seem to have reduced the gross consumption figure.
HDD12=2 (calendar month) vs 14 last September and 5 the previous one.
2016/10 146 (4.7/d) -205 (-6.6/d) 201 (6.5/d) 20160930-16:43-GMT to 20161031-17:12-GMT (31d) Meter readings: electricity generation 204.7kWh, export 164.3kWh, import 105.8kWh (thus net consumption (I+G-E) 146.2kWh); gas ~18.1m^3 (so 201kWh @ 11.1kWh/m^3).
Large tree to SE hard-pruned 2016/10/04 and may reduce 0800Z--1000Z grid-tie generation dip from shading (see after) this month and in spring.
Gas consumption higher than last year as OpenTRV on test this month.
HDD12=38 vs 32 last October and 15 the previous one.
2016/11 179 (6.0/d) -88 (-2.9/d) 599 (20/d) 20161031-17:12-GMT to 20161130-18:12-GMT (30d) Meter readings: electricity generation 88.4kWh, export 58.8kWh, import 149.1kWh (thus net consumption (I+G-E) 178.7kWh); gas ~54.0m^3 (so 599kWh @ 11.1kWh/m^3).
Lots of messing around with OpenTRV algorithms seems to have increased electricity and gas consumption over and above very cold month.
HDD12=154 vs 63 last November and 79 the previous one.
2016/12 180 (5.8/d) -88 (-1.7/d) 653 (21/d) 20161130-18:12-GMT to 20161231-18:29-GMT (31d) Meter readings: electricity generation 53.9kWh, export 32.0kWh, import 157.6kWh (thus net consumption (I+G-E) 179.5kWh); gas ~58.8m^3 (so 653kWh @ 11.1kWh/m^3).
HDD12=160 vs 38 last December and 183 the previous one.
2016 total (Jan--Dec) 1831 (5.00/d) -3739 (-10.2/d) 3851 (10.5/d) 20151231-19:20-GMT to 20161130-18:12-GMT (366d) Gas usage up due to cold winter.
Not clear why electricity use up again.
Note that grid electricity carbon intensity dropped significantly again in 2016, which would reduce the benefit of our electricity exports, but previous (0.43kgCO2/kWh) figures will be used for comparison.
kgCO2 787kg (2.2/d) -1608kg (-4.4/d) 732kg (2.0/d) ~-0.09tCO2 (~-0.24kgCO2/d) gas/electricity emissions total for 4 people, based on 0.43kgCO2/kWh for electricity (netted) and 0.19kgCO2/kWh for gas, ie slightly carbon negative.
Note that we haven't flown this year again, we still don't own a car and travel almost entirely by (usually-electrified) public transport.
2017 Daily PV Output


2017 data...

See the daily PV generation logs for 2017 as .csv and .png chart.

MonthConsumed kWh (/d)Generated -kWh (/d)Consumed kWh (/d) PeriodNotes
2017/01 207 (6.7/d) -71 (-2.3/d) 867 (28/d) 20161231-18:29-GMT to 20170131-18:44-GMT (31d) Meter readings: electricity generation 70.6kWh, export 41.1kWh, import 177.7kWh (thus net consumption (I+G-E) 207.2kWh); gas ~77.2m^3 (so 867kWh @ 11.1kWh/m^3).
HDD12=248 vs 173 last January.
2017/02 163 (5.8/d) -109 (-3.9/d) 497 (18/d) 20170131-18:44-GMT to 20170228-23:37-GMT (28d) Meter readings: electricity generation 109.4kWh, export 78.8kWh, import 132.4kWh (thus net consumption (I+G-E) 163.0kWh); gas ~44.8m^3 (so 497kWh @ 11.1kWh/m^3).
HDD12=139 vs 199 last February.
2017/03 166 (5.4/d) -300 (-9.7/d) 357 (12/d) 20170228-23:37-GMT to 20170331-16:29-GMT (31d) Meter readings: electricity generation 300.2kWh, export 244.3kWh, import 110.0kWh (thus net consumption (I+G-E) 165.9kWh); gas ~32.1m^3 (so 357kWh @ 11.1kWh/m^3).
HDD12=77 vs 162 last March.
2017/04 143 (4.8/d) -482 (-16/d) 117 (3.9/d) 20170331-16:29-GMT to 20170430-20:45-GMT (30d) Meter readings: electricity generation 481.9kWh, export 407.5kWh, import 68.8kWh (thus net consumption (I+G-E) 143.2kWh); gas ~10.5m^3 (so 117kWh @ 11.1kWh/m^3).
HDD12=65 vs 104 last April.
2017/05 149 (4.8/d) -528 (-17/d) 142 (4.6/d) 20170430-20:45-GMT to 20170521-19:54-GMT (31d) Meter readings: electricity generation 527.7kWh, export 450.5kWh, import 71.5kWh (thus net consumption (I+G-E) 148.7kWh); gas ~12.8m^3 (so 142kWh @ 11.1kWh/m^3).
HDD12=23 vs 23 last May.
2017/06 139 (4.6/d) -622 (-21/d) 109 (3.6/d) 20170521-19:54-GMT to 20170630-19:28-GMT (30d) Meter readings: electricity generation 621.7kWh, export 545.4kWh, import 63.1kWh (thus net consumption (I+G-E) 139.4kWh); gas ~9.8m^3 (so 109kWh @ 11.1kWh/m^3).
HDD12=1 vs 2 last June.
2017 running total (Jan--Jun) 967 (5.3/d) -2112 (-12/d) 2079 (11/d) 20161231-18:29-GMT to 20170630-19:28-GMT (181d) Implies total footprint to date -46kgCO2: electricity (average 223W gross) -492kgCO2e @ 0.43kgCO2/kWh (netted), gas 395kgCO2e @ 0.19kgCO2/kWh.

2017/05: Ecotricity sent our annual summaries for electricity and gas, stating that we had used just over 1.3MWh of electricity (~£270) and a ~4.2MWh of gas (~£262).

The gas figure is higher than usual, given that it was a cold winter, and ~3.5MWh would be more typical (maybe ~£250).

Given that we don't yet have a heat-pump, but that resistance heating is beginning to approach the carbon intensity of gas on the GB grid (currently ~300gCO2/kWh vs ~190gCO2/kWh), what would be the effect of moving from gas to electricity for all heat including water (DHW)?

At ~17p/kWh for electricity that 3.5MWh would be ~£600, but getting rid of gas and its standing charge and an annual gas boiler inspection/service would leave us 'just' net ~£250 per year worse off. Ie we might expect a £900/year electricity bill, rather than a £550 dual-fuel total (+~£100 maintenance).

Moving all our DWH consumption to solar PV that we'd otherwise export (eg with the help of a resistance-driven Sunamp store) would be roughly cost neutral, since the gas per-KWh cost is similar to what we are paid for electricity exports.

Even in the depths of winter, we export in electricity ~10% of what we consume in gas; a heat-pump and store could double that to ~20%, still requiring us to import the remaining 80%. In the shoulder months with a suitable Sunamp-like store we could probably cover all space heating. Without a heat-pump the effect on utility bills of using our own power locally is roughly neutral, though kind to grid in avoiding shipping that energy back and forth.

example 8-day UK electricity demand graph for 2007/10/11 c/o National Grid