Earth Notes: Saving Electricity (and Gas)

How we cut carbon emissions at home from 6 tonnes of CO2 per year to under 1t total, for all 4 of us!

Although solar electricity seems a glamorous way to do your bit for the environment and save money, probably far more effective on both counts is simply to reduce use and waste.

Every kWh of electricity saved avoids production of ~0.43kg of CO₂ in the UK (or possibly more realistically the DEFRA 3-year rolling average figure ~0.52kgCO2/kWh rising to ~0.6kgCO2/kWh at demand peaks as of 2008).

(Note that The United Nations Framework Convention on Climate Change (UNFCCC) allows wind farms to claim avoidance of 0.67kgCO2/kWh (0.67tCO2/MWh) of generation.)

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

My aim during this process has been to keep our current level of comfort, but do it with far less 'primary' energy (and thus CO₂ emissions).

June 2007

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 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 Jul 2008.) (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.

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.

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.

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 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).

Local Solar PV Generation

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.

Home Electricity Usage

Mains power consumption measured by power meter for select items:

Item	Power W (peak)	Notes
*Fridge/freezer	30W--40W (180W)	Is A+ rated: should use ~0.75kWh/day; but used about 0.9kWh on its first full day, and a little over 1kWh/day thereafter. ~80W max running the compressor(s); the ~180W peak measured may be part of the auto-defrost mechanism as no compressor was running.
Washing machine (Zanussi ZWD14581W front-loading 7kg washer-dryer)	2W 'off' (2kW)	See all our measurements in our review. Basically it looks like we can save up to 90% of the energy per wash on average (taking ~0.1kWh to ~0.2kWh) with care and by using appropriate modern detergents, though in practice we are using a mixture of 40°C, 30°C and cold washes, apparently saving ~0.5kWh/d in practice as of the end of 2009.
Dishwasher (Zanussi ZDS20101)	0W 'off' (2.2kW)	~0.8kWh for 'ECO' (cool, slow) cycle. Typically one load is run per day. ~0.02kWh to run a rinse, which with a little detergent I use as a surrogate pre-wash. ~100W most of the time that is it 'washing' without heating the water.
Slow-cooker (1.5l)	50W	Ancient model from my student days (so a modern one could probably be much more efficient); 0.36kWh (~7h) cooked a curry for our family of four.
*Central-heating and DHW gas-combi boiler	2W (20W--40W when CH is enabled)	When on 'standby', with just the timer running, ~1W is drawn. When producing hot water or central heating up to 140W is drawn, presumably at least in part for the combustion-assist fan. The central heating circulation pump alone seems to use 70W. The average power will depend on the duty cycle of the boiler, but maybe up to about 1kWh/day on a cold day; from 2008/09/22 16:30 for 24 hrs (a mild day for February) ~0.5kWh was used.
*Hands-free phone	3W--7W	Must be left on to receive calls. (Higher reading from PM230, lower from 2000MU-UK.)
*Bedside clock-radios	2W--7W	(Higher reading from PM230, lower from 2000MU-UK. We've turned off one of the clock radios.)
Bedside lights.	(48W)	One of the two bedside lights is still 40W incandescent: the other has been replaced by CFL (8W), but they're not used much anyway.
2 x Child's night-light (LED)	(0.5W each)	Device is rated 0.5W.
Cable TV box (PACE)	15W--20W	Standby mode consumes maybe 1W less. (Higher reading from PM230, lower from 2000MU-UK.)
DVD Player	20W	Draws little when 'off'.
TV (small, old)	55W--60W	Too old to have a remote or standby mode! (Higher reading from PM230, lower from 2000MU-UK.) This includes the power for a small separate SCART/UHF converter to allow use of a DVD player.
Gas cooker (electronic ignition and clock)	1W	1W standby with the clock active according to 2000MU-UK.
Microwave oven	3W (800W+)	3W standby with the clock/display active according to 2000MU-UK.
Electric kettle	0W (~2kW)	~35Wh (130kJ, or ~15g CO2 generating the UK grid electricity) to make a single cup of tea. Approx 0.5kWh/day usage when I am working at home!
Electric travel kettle	0W (1kW)	May be slightly more efficient for a single cup of tea, but more importantly the lower-power element is within the planned power of our early-2008 grid-tie solar PV install, and thus we may import far less energy or even none at all to boil that cup of water when the sun is shining in return for waiting slightly longer. My measurements suggest that if I boil exactly the water I need, and manually stop the kettle just before it comes to the boil, then I use under 30Wh for one normal mug.
Baby monitor (child unit, parent unit)	~5W total	~2.5W total when both are on 'standby'. Baby monitor out of use ~2008/01, back in use 2008/11 for newborn!
Dishwasher (Zanussi DW24)	0W 'off' (3kW)	Undetectably low load when not running even with 'on' lamp lit. 1.27kWh for prewash+wash on warm day, 0.86kWh for 'quickwash' which is now our usual programme. Unmeasurably small (with current meters) amount of energy to run a rinse, which with a little detergent I use as a surrogate pre-wash. Expired 2009/12/12.
Washing machine (Zanussi ZWD1260W front-loading 5kg washer-dryer)	2W--10W 'off' (2kW)	A few W (2000MU-UK reports 2W) even when apparently 'off' at the machine with the power light off, which is bad: definitely one to switch off at the wall. 0.74kWh for our typical full 40°C wash. 0.91kWh for a 60°C wash of towels with a 13°C mains inlet temperature (the manual specifies 1.10kWh, 63l and 145m for that wash, ie 0.22kWh and 12.6l per kg of washing; our replacement ZWD14581W is 0.17kWh and 8l per kg). 0.59kWh for a 30°C 'worn once' wash with a 10°C mains inlet temperature. 0.79kWh for a 40-minute tumble dry of a half-load. The water inlet valve takes about 10W when open, slowly turning the (front-loading) drum up to about 300W peak (<150W average) and up to ~600W peak while ramping up to high spin speed, and the heating element takes 1.7kW. Main bearings failed 2009/03/17!
Flashing fancy inside/outside light string	40W (100W)	Only on for a few hours per day and only for a few days per year.
Christmas tree lights (plain, incandescent)	40W	Only on for a few hours per day and only for a few days per year.
Old stereo system	20W	20W standby: consumption not measured when in use. Little used and now unplugged when not in use. FreeCycled 2008/07!
Child's night-light (incandescent)	(5W)	5W with light on; <1W in daylight should we forget to unplug it! Replaced with £2.49 LED night-light when bulb failed, and thus may just about pay for itself with saved electricity in a year or so.
*Fridge/freezer (circa 2000)	80W (190W)	0W minimum, +20W for internal light when door open. ~190W with compressor running. ~80W average measured over 1 day. This was our biggest single consumer of electricity in the house at ~2kWh/day. Replaced with A+ rated model 2008/03/03.
PC and monitor and printer	210W (250W)	The monitor consumes 30W on standby and 80W in use. Working the PC harder increases its power draw from 130W to 170W. Switched to a power-efficient old/used laptop from 2007/11 as the PC started to fail.

Items marked * need to be left on 24x7.

Study Electricity Usage

Mains power consumption measured by power meter for select items:

Item	Power W (peak)	Notes
*Netgear DG834G v4 ADSL router and WiFi	~8W	Up to ~10W with an extra LAN port in use. Down to ~7W with wireless disabled. May be OK to power directly from 12VDC, ie battery. 4W 'standby' consumed by mains adaptor when unplugged from the modem.
*Server	~4W (7W)	Low-power embedded "computing plug" for mail, Web and other servers. Mainly solar-PV powered (off-grid) thus reducing grid load still further. (3.5W minimum when quiet, max ~7W with mass storage active and CPU at 100%.)
Iomega 1TB USB 2.0 desktop external hard disc ("LDHD-UP", internally SAMSUNG HD103SI)	~8W	Adaptor not plugged into drive draws 0.9W from mains (not good). With drive on, ~8W idle or (gently) active. ~3W once 'ejected' ie put to sleep by the Mac.
MacBook laptop with LED backlight ("MacBook6,1", 4GB RAM, 250GB HDD, OS X 10.6.2)	~12W (40W)	Adaptor not plugged into MacBook draws <0.1W from mains (very good). Towards end of charging (~30m to go) with MacBook sleeping, draws ~20W, and with MacBook awake between ~30W and ~35W depending on screen brightness when otherwise quiet. ~12W once battery charged and working on a document WiFi running, ~1W sleeping with the lid down (after a burst to ~24W for ~20s while saving state to disc), ≤40W when busy (both CPU cores running flat out).
MacBook	~19W (60W)	Not run absolutely flat out, nor on lowest-power settings. ~3W with lid closed. Screen from 'off' to max brightness ~5W difference. [Disc drive died, probably from static damage, 2010/02.]
*Server	20W--27W (60W)	Low-power laptop/server for mail, Web and other servers. Partially solar-PV powered (off-grid) thus reducing grid load still further. (Higher reading from PM230, lower from 2000MU-UK when system/disc quiet but with all services running. Heavy disc activity adds ~3W according to the 2000MU-UK, and the is ~2W residual wastage in power supply when laptop 'off-grid', ie this mode cuts grid-power use by ~90%.) Replaced with ~4W SheevaPlug 2009/10.
ZyXEL P-660HW-T1 v2 ADSL router and WiFi	8W	May be OK to power directly from 12VDC, ie battery (unofficially ZyXEL confirms this but it would void the warranty). 2W 'standby' consumed by mains adaptor when unplugged from the modem.
*Cisco 828 DSL router	9W--14W	Safety rules and other reasons may require this to stay mains-powered. The higher reading is from the PM230 meter, the higher from the 2000MU-UK. Recycled and replaced with a combined ADSL/WiFi router ~2008/06.
WRT54G Wireless LAN	8W--12W	Looks like it could run directly from 12V DC supply, or could be eliminated entirely if the server-replacement laptop can double as a WiFi access point. Trying to remember to unplug it when no one is using it, maybe 12h/day. (Higher reading from PM230, lower from 2000MU-UK.) Recycled ~2008/06.
[*Servers]	[670W]	[Some very old equipment and an over-complex set-up. 220W for the Web server alone. Recycled having been replaced with <30W server/router 2007/07/22.]

Items marked * need to be left on 24x7.

Cost

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.)

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

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: http://www.earth.org.uk/saving-electricity.html

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: [http://ec.europa.eu/energy/] and http://concertoplus.eu.

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 300kWh	18
300 to 600kWh	26
600 to 900kWh	18
900 to 1200kWh	8
1200 to 1500kWh	12
1500 to 1800kWh	2
More than 1800kWh	11

From Net-Zero Electricity to Negative-Carbon

It has been pointed out to me that 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.

ESCo

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.

News

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 one child), or ~3t CO2 per year per adult.

After Starting Energy Conservation

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)

(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 nearby) for a base temperature of 12°C, lower than the usual 15.5°C to better reflect our usage, c/o DegreeDays.net.)

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

2008	Electricity		Gas
Month	Consumed kWh (/d)	Generated -kWh (/d)	Consumed kWh (/d)	Period	Notes
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 2008 daily PV generation logs 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.

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

2009	Electricity		Gas
Month	Consumed kWh (/d)	Generated -kWh (/d)	Consumed kWh (/d)	Period	Notes
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 2010 daily PV generation logs for 2010 as .csv and .gif chart.

2010	Electricity		Gas
Month	Consumed kWh (/d)	Generated -kWh (/d)	Consumed kWh (/d)	Period	Notes
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 mains 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 inclinded 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.
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 mains 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.

Sources/Links

• Saving-Energy.EU "is the new European portal for scientists and other people who want to contribute to a lower energy consumption. We want to give people a voice on the internet to help interesting ideas to be heard."
• Stern and the IPCC suggest (Times, 2009/02/01) that we need to get down to about 2tCO2e/year each. The article says "the scale of the challenge [...] is huge. At the moment humanity generates the equivalent of about 50 billion tonnes of carbon dioxide a year, roughly equal to eight tonnes for every person on the planet. There is, however, huge variation. Europeans generate 11-14 tonnes per head and Americans about 22 tonnes, while Africans typically generate 1-2 tonnes." So, for our household in 2008 our household primary energy (gas and electricity) alone was about 25% of our emissions 'budget'.
• Try i-measure or just in general Read Your Meter regularly to really understand your consumption: you can only manage what you measure.
• The UK's National Grid publishes real-time stats on power usage and frequency with the graphs showing a very distinct daily and weekly cycle with a somewhat counter-intuitive daily shape with peak demand ~8pm and minimum ~4am. See strange Christmas 8-day graph up to mid-day 20071227 (and 2009) with each day looking more like a weekend than a normal day, and with a strangely reversed morning-vs-evening peak pattern on Christmas Day (Tuesday 25th), possibly from people coming down early to open presents and put on the Christmas tree lights? Also see the little extra surge around midnight on 20071231 (and 20091231) presumably for New Year's Eve events! See also some sample August 8-day and 1-day National Grid demand curves, and some analysis of typical GB seasonal demand/load curves. Also see Christmas workers: National Grid.
• The NETA Balancing Mechanism Reporting System (BMRS) site is a useful source of real-time and historical UK grid-related data.
• Michael Bluejay's excellent guide to saving electricity; a little US-centric.
• The 2000MU-UK meter from Maplin.
• The PM230 kWh meter from EnergyOptimizersDirect.
• Another UK household's energy diet plan!
• See XePhi for Linux laptops that are tweakable for low-power running.
• Check your electrical power consumption with tools such as the Kill-a-Watt meter and those from Energy Optimizers Direct.
• The WhatYouCanDo electricity-consumption reduction story.
• Earthwhile: specialist energy saving store, which includes some useful items such as 'candle'-size CFLs and a 'standby buster' using <1W per socket.
• There are various other smart power-strips to turn lots of things off at once while avoiding bending down too often, or delving behind the sofa, such as from oneclick.
• Something "grid-interactive" like SMA's Sunny Backup or Sunny Island systems can provide the benefits of on-grid (for safe export of excess) and off-grid (the lights, fridge, etc stay on if the grid goes down) PV systems. In particular, a heating system such as a gas combi and/or heat-pump could keep running whereas normally they'd stop in the absence of grid power, keeping the house warm.
• Chris Barber of Wind and Solar Shop has been very helpful with pricing and planning, and I'd want to include him in any final bid/quote for work we get done.
• The Linux Laptop Mode Tools site.
• I am able to deploy my X-10 devices and Linux server to remotely disconnect unused items from the mains and/or run large loads off-peak and/or do some realistic burglar deterrence, providing that the X-10 controllers use much less power than the appliances that I am considering controlling (typically <2W for each X-10 controller).
• The Automomous House page claims that UK average household electricity consumption is 3MWh/year (presumably as of ~1994) from Boardman B et al (1995) "Executive summary" DECADE second year report Energy and Environment Programme, Environmental Change Unit, University of Oxford p2 (I see 4MWh+/year suggested elsewhere, eg BWEA ~4.7MWh/y, for circa 2006).
• Total UK electricity consumption is just short of 400TWh/year as of 2006/2007 (382.5TWh in 2006), eg Energy White Paper 2007: "5.1.8 The UK currently has around 76GW (gigawatts) of electricity generation capacity to meet annual consumption of about 350TWh (terawatt hours) and winter peak demand of about 63GW."
• Also see UK Energy in Brief: 2007.
• Alan Smart, energy operations manager for National Grid, is quoted 2007/09/06 by the BBC

  Typical evening demand this week across England, Scotland and Wales will be in the regions of 41,000 MW.

  If the Grid's analysts' predictions are more than 300MW out, the incident might be investigated, says the BBC. The piece also discusses big "pick-ups" from various TV events, such as 20020621 England v Brazil World Cup 2570MW, which is a lot of sulky cups of tea!
• See the BWEA's UK Wind Energy Database (UKWED), as of October 2007 showing ~2.2GW of operational wind generation in the UK, ie maybe 5% of peak demand.
• Each geographical/supply area and season may have its own characteristic load curve demand peaks and troughs, for example in Mumbai/Bombay the two peaks seem to be 0900--1200 and 1800--2200 (local time), with heavy load management demand curbing being brought in in May 2005 to reduce wastage 1700--2300.
• The French grid (RTE) publishes demand curves too.
• The Spanish grid (RRR) publishes real-time demand curves too.
• 2007/09/23: the BBC reports that investors urge action on climate change, including analysis from Lehman Brothers economist John Llewellyn showing that cutting waste should come early in the process or reducing environmental impact.
• As of November 2007 the BBC reports that "83% of respondents throughout the world agree that individuals will definitely or probably have to make lifestyle changes to reduce the amount of climate-changing gases they produce."
• One idea behind a global electricity grid or 'hypergrid' is to flatten out local peaks and troughs and have power 'follow the sun'. Many countries are already interconnected, and this global grid has the advantage of allowing injection of more RE/intermittent sources, eg solar PV in England could be powering the night lights of the far east of Russia 12 or 13 timezones away. A large interconnected grid eliminates at least some of the need to store lots of power from intermittent RE sources. Such a grid can be built regionally first, such as this European Supergrid.
• See maps of regional and national power grids and interconnections around the world.
• Power plants' CO2 levels revealed on the BBC News site, November 2007. "The UK's 192 million tonnes make it the ninth highest emitter, with per capita CO2 emissions of 3.2 tonnes." See the CARMA: Carbon Monitoring for Action site containing information on the carbon emissions of over 50,000 power plants and 4,000 power companies worldwide.
• It is considered good practice for electricity generation systems to have ~15% margin of supply over (peak) demand: at the start of 2008 Eskom of South Africa had as little as ~8% resulting in repeated load-shedding and blackouts.
• Some interesting fuel-energy-density figures from a 2001 Hans Blix talk:
  • 1kg of firewood corresponds to about 1kWh of electricity.
  • 1kg of coal and oil correspond to respectively 3 and 4kWh of electricity.
  • 1kg of natural uranium corresponds to about 50,000kWh of electricity.
  • 1kg of plutonium corresponds to about 6,000,000kWh of electricity.
  though he was arguing against solar/wind renewables in bulk! (Note also that the energy density of thorium (Th) may be about 1.8x that of plutonium.)
• As of May 2007 the UK's significant generation plant stock included, by fuel: 68 gas, 18 coal, 10 nuclear. In 2007Q3, of the 13Mt of coal burnt for electricity generation ~10Mt was imported. Source BERR.
• "DUKES": Digest of UK Energy Statistics (and the 2008 PDF from which some headline figures are that renewables generate a little under 5% of UK electricity at ~20GWh/y up a little under 9% from the year before with onshore wind the biggest increase, overtaking hydro for the first time).
• I gathered "heating degree days" (HDD) data from DegreeDays.Net for estimating underlying heat need/demand, to help normalise actual consumption and thus efficiency values. Incidentally, Martin of DegreeDays.net was very helpful over email too, eg with this 5-year block to December 2008 for my local weather station at London Heathrow (EGLL)!
• BBC story on safe disposal of broken CFLs, and the US EPA's advice too.