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This is related to Relative carbon footprint of gas or electric heating (UK) but with 2 key differences: things have changed in the 7 years since it was asked; I'm asking about the use of the systems I have, rather than about installation.

I've currently got a gas boiler (condensing, supposed to be well over 90% efficient and set to a return temperature that should maximise efficiency). I also have the option of heating some areas with resistive electric heating (bathroom underfloor heating or standalone electric heaters). There's also a gas fire in the living room, but I don't spend much time there.

There's clearly a benefit to heating only the parts of the house that are used - I already have the radiators turned way down in the spare room and off in the utility room. Using the electric heaters is good for heating only the rooms that are needed, e.g. I like to sleep in a cold room but my daughter doesn't.

What's less clear is when I start heating several rooms electrically. I'd like to estimate the carbon difference between heating some of the house with electric heaters and most of it with the central heating (I can guesstimate the difference in external surface area of heated rooms). So given a typical UK winter generation mix and the inherent inefficiencies of transmitting electricity, how does the carbon footprint from resistive electric heating compare to that from gas?

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We have to remember here is that all electricity have the emissions of marginal electricity. So, if there's only a single gas-fired power plant running, that's the last plant profitable as electricity price rises and the first plant unprofitable as electricity price falls. Thus, if you can reduce your electricity consumption, that's the plant that will reduce its output as a response to you reducing your electricity use.

I assume that at least on non-windy hours, there will be gas-fired power plants running providing marginal electricity. However, due to the massive installation of wind power, if it's very windy the gas-fired plants may be turned off.

Therefore, I claim that most of the time electricity has the emissions of gas-fired generation. Gas-fired generation have an efficiency of 50% (actually it's slightly over 50% in state of the art, but there will be transmission losses so let's say 50%), as opposed to your gas boiler having an efficiency of 90%. Therefore, it makes sense to use the gas boiler.

If you are only concerned about emissions, you need some way to see if the marginal electricity is provided by gas-fired plants or by wind. If marginal electricity is provided by gas-fired plants, then electricity spot price will be about twice the cost of gas per megawatt hour (because of 50% efficiency). If marginal electrictiy is provided by wind, then electricity spot price will be whatever the cheapest load that can be increased on demand is willing to pay. In most cases, this will be some factory or a bitcoin mining operation. There may be some online service that tells the momentary generation mix. The key to look as is whether a single gas fired power plant is on. If even a single gas-fired power plant is running, that's the plant that will reduce its emissions as a response to you reducing your consumption.

However, there are several things to note. Firstly, as more and more wind power will be installed, there will be less and less hours when marginal electricity is produced by gas. Secondly, in the future gas-fired plants may capture the carbon dioxide, but I'm sure your gas boiler won't capture its emissions ever. Thirdly, more transfer capacity from Norway may be installed, and this may allow using Norwegian hydropower as the marginal electricity production method. Thus, my assumption is that over time marginal electricity will become cleaner.

Also, as you correctly noted, with electricity it's easier to fine-tune the heating of individual rooms.

So, I propose that you should make the decision on total cost. If the total cost of burning gas (and any maintenance the gas boiler might need) is greater than the total cost of providing resistance heat, use electric resistance heaters. If electricity is more expensive, use gas. In most cases, this will result not only in the cheapest heating, but also in the most environmentally friendly heating. Also you can consider in this comparison the costs of heating only needed rooms with electric resistance heat and the costs of heating the whole house with a gas boiler.

Also do consider split air source heat pumps that convert outdoor air heat into a higher temperature. They have a high installation cost but the investment may be useful as it reduces electricity consumption of heating to one third of resistance heating consumption. They also allow fine-tuning heating of individual rooms if you install a heat pump in every room, but of course a heat pump in every room will cost a lot.

Air-to-water heat pumps and ground source heat pumps may be an option too, but their investment cost is higher and in some cases it may be non-permitted to drill a geothermal well if there are underground constructions or plants to do such construction, or if the geothermal well could contaminate water supply.

With heat pumps, almost always electricity wins over operating the gas boiler.

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  • Very thorough, thank you. A few points in brief to your implied questions: Electricity is considerably more per kWh here to the consumer; the boiler is also needed for my hot water and baseline space heating so the marginal cost of maintenance is minimal (and the rest of the system I maintain myself); The go-to option for a house that already has wet central heating is an air-to-water heat pump. While not a drop-in replacement it's less invasive and cheaper than a unit per room.
    – Chris H
    Dec 7, 2021 at 21:16
  • There is indeed a tool: GridWatch. This shows that for the last few weeks there's always been gas burning for power. Rather strangely it says the same during summer, and I recall reading that that wasn't the case. It may be an artefact of the display. Worse still, there's even coal being burnt, even though the gas generating capacity isn't maxed out (some of that may be about where the power is needed). So on balance any savings would have to come from me heating less than about half the house (on a carbon basis) or less than about 1/3 (price ratio basis)
    – Chris H
    Dec 7, 2021 at 21:23
  • One further note - here at least it's not quite true that the marginal generation comes form the cheapest available source. That would be true in a real-time free market, but the grid operator and regulator set up contracts to prioritise reliability of generation, rather than momentary cost. Thus suppliers are guaranteed to sell a certain amount so they can afford to keep their generators available
    – Chris H
    Dec 9, 2021 at 10:58

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