I'm in the process of switching from a gas combi boiler to an unvented hot water cylinder for our DHW (domestic hot water) needs. I'm an OVO customer using both Green Gas and Green Electricity (Economy 7), and I'd like to minimise the environmental impact of my energy use.
Unvented cylinders come in two flavours - direct, which are heated directly by heating elements, powered by mains electricity; and indirect, which are heated via hot water coming from a gas-powered boiler. (There are other options, but these are the two that I"m looking at).
On the face of it, a direct cylinder should be better for the environment than an indirect one, because electricity comes from nice things like solar panels and wind turbines, but gas is a fossil fuel and evil. But:
- Green Gas uses (some) biogas, so isn't all evil
- If using electricity, I'd be heating the water between midnight and 6am, so presumably none of that electricity is going to come from solar panels
- Gas is allegedly "more efficient" but that seems to be more about £ per Kw than anything else.
- Electricity production isn't perfect, with greenhouse gases used in high voltage equipment, etc.
(for what it's worth, whichever option we go for, I'm planning on adding solar thermal panels, to minimise the amount of gas/electricity that we do draw on).
In general, it seems hard to find a direct comparison between the two options, so I'd welcome any thoughts/facts that people can add.
At the moment I'm still erring towards using electricity to heat the water, but I'd welcome thoughts on which is "better" environmentally. I know that electricity is going to push up my costs a bit, from something like £30 pcm to £45pcm but I'm prepared to pay for something that's better for the planet... if it is better!
Best answer by dante
Thanks for all of your replies, you've given me a lot to think about. I've also done more digging, and thinking, and I think I'm beginning to get a clearer idea of the options I have. Here's what I've found out so far:
Q1) Is night-time electricity greener than daytime electricity?
Yes, this certainly seems the case. I had a good poke around the OVO website, and found statements like the Vehicle-to-Grid charger "will charge your vehicle battery with electricity from the grid when electricity is cheap and more likely to be of a lower carbon intensity", and the Smart Charger which will "reduce your charging emissions by up to 22%" ... "Savings based on Average Grid Carbon Intensity (gCO2/kWh) and by moving three-hour charge window from 5pm to 11pm."
But the big find for me is the Carbon Intensity website which maps out the carbon intensity of electricity generation in the UK: https://carbonintensity.org.uk/ where you can clearly see the CO2 impact dropping on a nightly basis throughout the UK.
I can't find any suggestions that UK electricity is more environmentally friendly in the daytime than at night, so I think that one is answered.
Q2) is night-time electricity more or less green than a combi boiler, if used to heat an unvented cylinder for DHW?
According to that Carbon Intensity website, emissions from electricity generation drops to ~100 grams of CO2 per KWh at night, although there seems to be a massive regional variation, with parts of Scotland generally up the top with 15 and parts of Wales generally bottom with 350.
To compare this with domestic boilers, I found a 2016 Carbon Footprint of Heat Generation paper published by the UK Parliamentary Office of Science and Technology. This says that "New boilers are required to have ... a carbon footprint of 210-230". (I'd quite like to find an exact figure for my boiler, but can't. I may email the manufacturer).
Using these figures, Scottish electricity is 14 times "greener" than my combi boiler, but Welsh electricity is 50% worse. So if I were in Scotland, it would appear to be a no-brainer, but as my region is towards the bottom of the list, it's not so clear cut.
I also haven't taken into account some other factors, which could push it one way or another in a close competition, like:
- I've assumed that both forms of heating are as efficient as each other. However, as
@PeterR1947points out, some of the heating from the boiler will be lost as the water makes its way to the cylinder, whereas the electrical losses should be near zero.
- Some of the gas will "green" biogas, so presumably those emissions don't count, but I don't know if biogas is part of the mix of gas that is piped to my property, or if like the Scottish/Welsh electricity divide, it's other people elsewhere that have access to that energy source.
Overall, I haven't yet reached a conclusion here. I also feel that, if I'm consuming electricity on a green plan, then I'm driving demand for green electricity, which should encourage suppliers to invest in renewables. If I'm consuming gas, then I'm not helping shift the demand from electricity to gas (a bit like if I fly but carbon offset my travel, I'm still paying a company that's going to make a profit and use that profit to fly more planes and expand more airports and promote air travel, none of which my offsetting will offset). But, for energy supply, I don't know how valid this argument is; perhaps OVO will use the profits from my gas supply to fund renewable electricity? But it seems unlikely that the wholesale gas supplier will.
If anyone has any other thoughts or opinions then please do share them. Rather than make this reply any longer, I think I'll reply separately to individual comments above, but again thanks you for all of them, they are all very helpful.