Gas vs Electricity for Heating Hot Water

Hi - I’m not sure where to start, but I am trying to figure out at what price it is cheaper to heat hot water in an immersion tank with electricity, rather than gas? I should be able to do this overnight at 5p per kWh via Octopus Go for 4 hours overnigh, but am struggling to get my head round how this compares in energy terms to gas…
Don’t get me wrong, I want to switch, but I just want to know if its going to be cheaper/the same/more in £££s?

This is a tricky one. Your boiler will have a published efficiency level. That figure is for a highly tuned model with the temperature set to the optimum for that model. i.e. you won’t get that efficiency. Then there is heat loss from the pipes, inefficiency of the heat exchanger in the cylinder, the electricity to control and pump the system. It is unlikely that you are getting better than 50% efficiency and possibly much less. Assuming 50% you can easily calculate the effective cost of heating your water as cost of gas x2. If that is more than the cost of off peak electricity then you should save money by switching.
If you can also disconnect the gas as a result then you will also save the standing charge. Octopus will do that but most suppliers either won’t or charge so much that you would never recover the cost.

Gas will still be somewhat cheaper. The unit cost of gas is typically 3p/kWh.
Lets say the efficiency of a gas boiler is ~85% on hot water this effectively works out at 3.5p/kWh of hot water delivered. So at 5p/kWh for electricity it will still be just slightly cheaper with gas.
This equation changes if you are using a heat pump (rather than an immersion element).

Don’t forget to take into account the standing charge. Octopus GO charges 25p a day whereas my current Octopus Flexible gas only charges 19.64. which is equivalent to an additional 1.8kWh of gas a day.


Apologies for the delayed response…
There are lots of variables to this. I have done this, and have found with my boiler and tank it is cheaper for me to heat my water with electric during my Octopus Go period.

To fire our boiler and bring it to temperature, it seems to use a lot of gas, once it’s running it’s consumption drops. Our hot water cylinder is quite small and the immersion heater can bring it up to temperature in under an hour.

As an example; yesterday, I heated our tank from 26.7°c to 42.6°c in half an hour using our gas boiler. This was a temperature rise of 15.9°c and according to Octopus used 0.25m^3 of gas (2.79kWh).
Using the data pulled from HEMS, on a previous occasion the immersion heater has been able to take the water temperature from 33.4°c up to 52.3°c over a half hour period. This works out a 18.9°c increase and used 1.49kWh. This works out at 8.06p in gas and 7.09p in electric (Octopus Go rates) for a similar(ish) rise in temperature.

If we had our central heating on, then it would probably make more sense to heat it with gas, and outside of our Go period it would also be cheaper to heat using gas.

Also, our boiler is set to a 65°c output, so as the tank temperature get closer to 60°c the rate of heat transfer drops - whereas with the immersion heater we see a fairly linear temperature rise right up to 60°c. This means that the gas boiler would end up running for a longer period than the immersion would need to.

I prefer to use the immersion heater as we save a little money during the summer, but also the electricity from Octopus is renewable.