Earth Notes: Towards a Real LZC Low/Zero-Carbon UK Home (2007)

Updated 2024-06-15 10:10 GMT.
By Damon Hart-Davis.
Can we really go zero-carbon or negative-carbon in an existing home near London? #lowCarbon #home #microgen
650Wp 1kWhPerDay sim
We have already taken steps to reduce waste of electricity and gas, since conservation is the cheapest and simplest measure to start with. But how low can we go?

(Article written/started 2007-12-08; we became become a SuperHome in early 2012 with more than 60% energy/carbon savings over an unimproved home.)

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Various carbon calculators already rated our 4-person household as pretty good. (Only just over 1t of CO2 per person per year compared with a national average nearer 2.7t according to the [archive] Google UK Carbon Footprint Project tool as of December 2007.) And our mains/grid electricity (7kWh/day) and natural gas (10kWh/day for DHW, ie hot water, and in winter an additional 20kWh/day CH, ie space heating) seem also to be comfortably below averages reported by the government, etc.

Extra Layer of Loft Insulation

We had an extra layer of insulation (~10cm) put in the loft (December 2007) which should keep upstairs a little warmer and reduce wasteful heat loss. "Part L" building regs suggest a minimum of 27cm, and we come up to that level in June 2008, part subsidised by our gas suppliers, if current plans pan out.

We're also on an 'all green' electricity supply possibly saving 1.1t CO2/year if it is really is all from zero-CO2 sources every hour of every day.

We have a little more we can do in the way of heat insulation (eg behind radiators on outside walls) but we have probably got to actively generate/collect some RE (Renewable Energy) to get much lower if we don't want to sit in the dark and cold singing mournful whalesong...

The possible targets that I have in mind are:

  1. To get our house electricity-neutral year-round (eg so that we export as much electricity as we import), almost certainly solar PV.
  2. To cover most of our DHW (hot water) demand from locally-captured RE, probably solar thermal.
  3. To cover all or almost all our heating (DHW/CH) demand from locally-captured RE, probably solar, possibly with a seasonal thermal store and/or GSHP.
  4. To get our house grid-independent year-round (eg we export on all but the darkest mid-winter days) and can run critical loads such as lighting and heating controls/pumps even if the grid is down.
  5. To export enough energy that we compensate for some or all of our energy consumption away from the home, eg at work.

We may not be in our current house for very long, but if we are, there are a few problems with improving its energy performance. For example:

It seems to me that whatever we do it should be modular, so that we can build the system piece-wise when we have resources, even though this implies some extra expense in scaffolding etc, from multiple visits to fit equipment.

Also, the current UK RE regime does not include many significant/realistic financial inducements to install microgeneration such as a feed-in tariff, so achieving the above targets by degrees allows the possibility of putting off parts that might get better support in future.

Whatever we do, it will be for the benefit of the environment rather than to line our pockets.

Year-Round Zero-Electricity

Almost certainly the easiest step independent of the others would be to go electricity-neutral year-round, ie make as much electricity over the course of a year as we use. This probably is not zero-carbon for various reasons, but it is a good start. The maximum that a simple (single-phase) grid-tied solar PV G83/1 system can export is 3.7kW (16A) without special arrangement with the local DNO (Distribution Network Operator, ie the owner of the local power cables), so we could probably usefully put up a ~4kWp system.

(Some islanding/battery capacity could ensure that we avoid drawing power from the grid during the 4pm-to-8pm mid-winter peak when generation is most carbon-intense and is competing for gas with domestic heating, as well as maintaining our lighting and other essentials in a power-cut.)

Energy analysis

This ~10kWh/day year-round RE average would be the equivalent of about 37% of our 2007 average daily grid energy imports (27kWh/d electricity/gas year-round average).

CO2 analysis

The most favourable/optimistic analysis of the CO2 saving this would make is to (a) ignore the time for the equipment to pay back manufacturing CO2, and (b) to assume that the grid acts as a perfect storage medium such that importing/drawing 1kWh (eg on a winter's evening to power lighting) is exactly balanced by exporting 1kWh at a different time (eg at summer noon), and that exporting excess defers use of carbon fuels for another grid user. Under this analysis we can assume that every kWh of electricity saved/exported avoids production of ~0.43kg of CO2 and that we can ignore ignore other transmission (etc) costs. In London, 1kWp of PV might produce a year-round average of ~9.9kWh of power (assuming 2.47Wh/Wp/day from south-facing optimally-inclined fixed panels). Our reduction in CO2 per day is therefore 9.9kWh*0.43kg/kWh, ie ~4.25kg/day or 1555kg/year, taking us to a net negative CO2 for electricity of 452kg/year, ie we are saving nearly half a tonne per year of CO2 that would otherwise have been produced generating power for our neighbours.

A more pessimistic view of the CO2 savings would start by discounting the manufacturing energy costs from the total, ie maybe 5--6 years amortised over a useful 25--30 year average equipment life. Panels often have 20+ year live guaranteed, grid-tie inverters more like 5, and lead-acid batteries may need replacement every 3--10 years for example. So, from the ~9.9kWh/day leave ~8kWh/day energy after manufacturing.

Then, conservatively, we should assume that an exported kWh may displace only the most efficient (natural gas) grid generation sources, maybe at a little over 0.2kg/kWh (not allowing for transmission losses). Thus, a light on on a mid-winter's evening when not powered by our PV costs maybe twice the CO2 that would be saved by powering that same light from PV instead of using minimally-carbon-intense non-baseload daytime grid power. And except at noon on bright non-winter days, a 4kWp array is unlikely to be able to cover the instantaneous power draw of the heating elements of appliances such as our kettle, washing machine and dishwasher. Let us assume for the moment that the year-round average mismatch of generation and consumption like this is (say) ~50%, ie that we will have to import 50% of all the units we use, even though that leaves more units to export each day.

We'll ignore transmission losses on exported units, assuming that near neighbours will consume whatever we export, especially since we did not allow them in our 'least-intense-displaced' figure.

Instead of importing all our 2.6MWh/year at 0.43kg/kWh (1.1t CO2), we import half (producing 0.55t C02) and have 4.5kWh/day (1.6MWh/year) to export saving generation of maybe 0.33t CO2 on behalf of our neighbours, so a total CO2 saving of ~0.88t CO2 per year.

In principle, given that we are already on Ecotricity's 'all green' traffic, our average kgCO2/kWh figure for imported power should be lower, even zero, if we're really getting 'green' power round the clock (eg solar/wind/hydro). That would mean that our 'saving' would only be the figure that we save our neighbours by making their electricity more green. That would be our entire average 8kWh/day (2.9MWh/year) at ~0.2kg/kWh, ie ~0.58t CO2 output avoided each year.


ItemBenefitCost £/otherFuel Savings/Earnings £patCO2pa saved min/max
4kWp Solar PV Takes us electricity neutral (zero-energy, not zero-carbon). £28k (ie ~£7/Wp) + ~28m^2 of south-facing roof space £600pa from reduced import and selling excess power and ROCs 0.58/1.55
1 day's 'essentials-only' battery backup (~5kWh) Minimises power draw at peak grid demand and thus carbon intensity, and gives power-cut protection, eg for lighting and other essentials. Might also allow fast/frequency support to National Grid. £1k to be replaced every 5 years approx ? support fees. ?

Solar DHW (SDHW) and some CH Support

The next logical step would be to put in an oversized solar thermal system to cover most DHW (hot water) demand year-round, even in winter, with overheat protection in summer, possibly using a drainback system, (near) vertical panels, and some overhang shading for summer. Normally these systems are designed to just provide all the DHW in summer and thus maybe only 20% to 30% in winter; I would like a higher winter fraction and thus summer overheat protection would need to be better than usual. The oversized system should include provision to attach the CH radiators and/or future underfloor radiant heating and other heat sources such as GSHP. (It would be unfortunate if we could not retain our existing reasonably-new and efficient gas combi as topup/backup for DHW and CH for the winter.) One datapoint suggests that with care maybe 67% of year-round DHW, and some CH support) could be provided by solar thermal this way.

I would prefer the solar thermal pumps to be driven by directly-attached PV, both to eliminate any parasitic mains electricity use, and also to ensure that the system need not stop if there is a power outage.

Note that if we wanted to expand this later to CH (central/space heating) then we'd need to be able to add more capture area on our small roof and so we'd need to ensure on the initial installation that there is space left (given possible competition with space for solar PV too) and that the water volume in the collectors remains sufficiently small to allow efficient energy capture in winter; both of these suggest selecting evacuated tube solar collectors from the start.

Energy analysis

This ~10kWh/day year-round average RE average would be the equivalent of about 37% (without a seasonal thermal store excess energy in summer is not used) of our 2007 average daily grid energy imports (27kWh electricity/gas year-round average).

CO2 analysis

If this were simply to cover all DHW heating (~10kWh/day) year-round, which should be possible with 13m^2 at somewhere between 70* and vertical, that would avoid production of ~0.19kg of CO2 per kWh of heat from burning mains gas, ie ~0.69t CO2 per year.

Conceivably this might also contribute to CH (space heating) in autumn/spring, which might save another 10kWh/day for 3 months, bringing us to 0.86t/year.

In this case we can assume that the manufacturing energy cost is paid back very quickly, in a couple of years typically, and so can be neglected.

ItemBenefitCost £/otherFuel Savings/Earnings £patCO2pa saved min/max
Solar Thermal DHW+ Saves ~50% of mains gas use for heat ignoring cooking (reduced carbon footprint), covering DHW all year round. Maybe £10k + 13m^2 of south-facing vertical wall £120+pa reduced gas bill at 2007 prices 0.69/0.86

Space Heating: GSHP and Seasonal Thermal Store

In theory, and ignoring cost, we can safely (ie no overheating problems) have as much solar PV as we like, and export or simply not use any excess. Supposing that our solar thermal generates enough on average per mid-winter day to cover DHW (10kWh/day), then we could cover the balance (CH at 20kWh/day) with a heat-pump (GSHP and from a moderate seasonal store) of CoP>3 and ~6kWp of extra solar PV (generating a mid-winter 6kWh/day).

If we wanted to export all or most of the excess electricity in summer then we'd probably need to upgrade to a 3-phase supply, at which point a G83/1 grid-tie inverter can export a little over 11kW maximum. A little more might be available by negotiation with the DNO.

If we're covering ~67% of year-round DHW/CH demand directly from solar thermal then we might be able to size our thermal store to maybe 25% of the total, allowing for some greywater recovery, and using the area around the water tank and an extension of the thermal store. That implies maybe 20kl (20t) of tank. That's almost small enough to go in a cellar in the house, and a slow leak of heat back into the house in winter might be welcome providing it can be minimised in summer.

It would be better to use underfloor radiant heating (UFH) than radiators in conjunction with the heat-pump to keep the CoP as high as possible.

Energy analysis

For 6 cold months this would replace ~20kWh/day of imported gas for CH demand, and for the remainder this would probably export ~20kWh+/day of electricity. This ~20kWh/day year-round average RE average would be the equivalent of about 75% of our 2007 average daily grid energy imports (27kWh electricity/gas year-round average).

CO2 analysis

The CO2 savings on the the gas side are fairly simple: if the system works as intended then it should eliminate the need for mains gas for space heating (except possibly for a few exceptionally bad days). That would therefore save 20kWh/day * 0.19kg/kWh * 180 days = 0.68t. We must be careful not to double-count any of the savings from CH-assist from the SDHW 'module'.

If we assume that exported power (when CH is not needed) is only displacing minimally-intense grid generation then we have might save (say) 20kWh/day exported * 0.20kg/kWh * 180 days = 0.72t. That figure does not explicitly discount manufacturing energy costs, etc.

ItemBenefitCost £/otherFuel Savings/Earnings £patCO2pa saved min/max
Additional 6kWp Solar PV to Drive Heat-Pump Eliminates grid gas use for heating (takes us zero carbon). £42k (ie ~£7/Wp) + 3-phase connection + ~42m^2 of south-facing roof space or south-facing ground, though possibly less if angled at 70°+ for maximal winter efficiency. £500pa ~4MWh in summer exports at ~£0.13/kWh. 0.72
GSHP Multiplies heat gain from solar PV (takes us zero carbon). £7k + maybe £3k for collector pipework and civils £120+pa reduced gas bill at 2007 prices 0.34
20kl Seasonal Thermal Store and Greywater Heat Recovery Improves CoP, possibly to 5 or better (takes us zero carbon). Maybe £10k including tank and equipment and civil engineering. Improves performance of GSHP above. 0.34

Going Electricity-Negative

Adding a final 4kWp of solar PV to the original 4kWp that took us electricity-neutral, plus enough battery store for maybe a week, should ensure that we never needed to draw electricity from the grid (except in the the most long, gloomy and unpleasant stretches of weather) even mid-winter, so that we would become pure exporters (or zero) all the time.

Energy analysis

This ~10kWh/day year-round average RE average would be the equivalent of about 37% of our 2007 average daily grid energy imports (27kWh electricity/gas year-round average).

CO2 analysis

Assuming the same assumed energy output per year from 4kWp (2.9MWh/year) as for our first 4kWp, but assuming that all of this is exported (and used by near neighbours, so with minimal transmission losses), then our low CO2 saving figure is based on our assumed displaced least-intense 0.2kg/kWh figure, and our highest estimate at the standard 0.43kg/kWh, ie in the approximate range 0.58t to 1.3t.

Using our battery bank to ensure that we never need to import even mid-winter when grid demand is highest and generation probably most carbon-intense (providing that our inverter can handle the highest (heating) loads), allows us in principle to claim the full CO2 savings from our first 4kWp too. If we assume that any imports on our 'all green' tariff are already zero-carbon, the gain is just that of minimally-intense generation from all our PV power, which is already the minimum saving claimed.

Note that using energy via batteries, ie charging and discharging them, itself wastes some energy, maybe as much as 20% of that used in charging. Thus 8kWp of panel, after battery losses, may only just carry 7kWh/day load, leaving nothing at all to export mid-winter.


ItemBenefitCost £/otherFuel Savings/Earnings £patCO2pa saved min/max
Additional 4kWp Solar PV Eliminates electricity import from grid (takes us negative-carbon). £28k (ie ~£7/Wp) + ~28m^2 of south-facing roof space £600pa eliminated imports and ~3.6MWh in exports at ~£0.13/kWh. 0.58/1.3
1 week's battery backup for bad winter weather (~50kWh) for above Ensures power use is maximally netted, and gives power-cut protection. Might also allow fast/frequency support to National Grid. £10k to be replaced every 5 years approx Possible £1000+pa in support fees and/or ability to export at peak times in future for a better price per kWh. ?

Doing It All

So what would be the capital cost (and other resources) of doing it all, and essentially going off the electricity and gas grids for heating?

ItemBenefitCost £/otherFuel Savings/Earnings £patCO2pa saved min/max
Solar DHW, CH, and PV Going carbon negative by ~8MWh of electricity (3.4t of CO2) per year. £130k+ (made up of £100k solar PV (+100m^2 of south-facing roof and/or unshaded ground) + £10k solar thermal (+13m^2 of south-facing wall) + £20k GSHP and seasonal thermal store + £2k/year in batteries) £1k in savings and fees at current rates. 3.2

At current retail (and reselling) energy prices, and without external incentives such as government grants, this is clearly not financially viable given typical 25 year equipment life, but that is not the motivation.

If this scheme is viable, and once it has recouped its embodied energy, this would be taking our CO2 footprint at home from +1t/year to -1t/year each, ie firmly negative, and might well cover some of our other activities.

Other than the capital cost (ie upfront cash), the main limitation is probably the collector area needed, which needs to be mainly unshaded and south-facing in order to maximise efficiency.

Energy analysis

We'd go from an import of ~27kWh/day to an export of >~20kWh/day on average.

CO2 analysis

If we applied all the above mechanisms we might reduce our household CO2 output each year by a little over 3t by a reasonably pessimistic analysis.

It is fairly clear that the most cost-effective component to reduce CO2 emissions per £ spent is probably the solar DHW, so possibly that should in fact be the first 'module' to be implemented. It can be constructed so as to continue to produce hot water even in the face of failure of mains electricity (and gas), ie be completely autonomous with pumps driven by small dedicated solar PV, and depending on the details of the construction and plumbing might mean that we could divert a little heat to keep the worst of the chill off the house which could be particularly valuable with children.

Location, Location, Transport

We would like to move to somewhere close to a major train station with a fast London connection. To get enough space for the above LZC scheme we may have to move a little further away from public transport links/termini, and so we might need a zero-emission vehicle such as an electric scooter or car to use for commuting, local shopping and general transport, if no longer possible on foot as now.

The costs (and charging) of such a scooter/car should be part of the scheme. For example, a new G-Wiz i at December 2007 prices is about £10k with about 40 miles range on a little under 10kWh, so would be tough but not impossible to accommodate within the above scheme mid-winter (a full charge is more than 1 day's electricity or DHW energy) but would probably be a breeze in the summer with solar PV though G-Wiz's charger "needs up to 12Amps available to work properly" according to their engineers, so still might force some power import. The G-Wiz can accommodate 2 adults plus a child and (for example) shopping. (A second-hand G-Wiz is ~£5k.)

There are parking and charging points in many of the main places that I would go to on business around London. From my home to my main client is ~16 miles, well within the car's range, especially given the availability of a charge point at the client's site.

Another electric car is the [defunct: ""] MEGA City with a similar range for a similar price, and there's an [defunct: ""] electric scooter with a 70 mile range for about £7k. So there is already a bit of choice and infrastructure available in London. I did a test drive.

Since the car's batteries represent a little over 1 day's home electricity, it is just conceivable that they could be part of the scheme's battery bank, though G-Wiz has said that direct access to the 48V is not approved of:

"If someone accessed the 48V under the back seat, they would void the warranty and you couldn't monitor if they pull too much current out of the batteries and damage them."


Water is another limited resource, and takes energy to treat and bring to us, drinkable, via the mains, and take away again as waste. (The carbon footprint of 1l (one litre) of UK potable mains water is estimated at 0.298g of CO2.)

Using uSwitch's calculator I estimated our total household consumption (2008-02-11) as ~125m^3/year (including ~33m^3/year each in bathroom and kitchen). That may be a significant under-estimate at ~340l/day, but I'm guessing that the biggest elements are my bath (~75l/day), other showering/baths (~75l/day), washing machine (~60l/day), the dishwasher (~20l/day), and loo flushing (~20l/day), totalling 250l/day, so it's plausible. The Thames Water meter tariff as of 2008-02-11 has fixed charges of £67/year for water and waste, and then usage charges of £1.48/m^3. Compared to our current £280/year bill we could be 20% better off metered. In other words, I suspect that we're already reasonably efficient.

Done and To-Do

While it may not be possible to go completely zero-carbon, there are things that we have already done and more that can be done in our existing small property in suburban London. (Note that staying small and improving an existing building is quite 'green'.)

Some energy-saving things already done:

Non-energy things done:

Some possible further efficiency improvements to make, highest-priority/easiest/in-hand first:

Non-energy things that we might do include:

(UK per-capita annual water consumption is ~150l circa 2009, with the government's strategy to reduce this to ~120l/p/y by 2030, using efficiency measures, metering and tariffs. Roughly one third of UK households are currently metered for water. The south-east of the UK, eg around London, is significantly water-stressed. 1% of all UK CO2 emissions can be attributed to the water industry.)

Also see the equipment change notes.

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