What happens when it gets cold outside?
When the external air is colder, the heat pump is less efficient. It still heats everything to the required temperature, but it takes a bit more power to do it, and because it’s cold we need to heat the water to a higher temperature – a double whammy! There is a very good correlation between the temperature outside and the power that our heat pump uses over a 24 hour period:
The eagle-eyed among you will spot that the day that the temperature dropped to -8oC, the energy usage is a little below the trend line. That’s because (as set) our heat pump climate curve only went down to -3oC, and so at -8oC we were a little under-heated. Had the low temperatures persisted, I could easily have adjusted that.
How much did it cost?
If I assume an electricity price of 34.04p/kWh and a gas price of 10.33p/kWh, how did we do?
I have compared our monthly electricity usage in heating our house this year with our monthly gas usage over the last year that we had gas (2020-2021).
To breakeven, we have to use around one-third of the electricity than we would have used gas. Most of the time we do, and over the whole year we will be about £400 better off than had we been on gas. However, as it gets colder the heat pump is less efficient, so it does cost more in the winter and less in the spring and autumn. This has been a cold winter, and we have spent £60 more heating our house over the last three months than we would have spent on gas. However, as soon as the weather warms up, the heat pump gets more efficient and we expect to more than recover this overspend over the next couple of months.
So Why Have a Heat Pump?
A saving of £400 a year doesn’t pay for the additional cost of installing a heat pump, so why do it?
For the future of our children! Our heating and hot water now uses about one quarter of the energy that we previously used. That’s a massive 2.5 tons CO2 that we are saving each and every year, and this figure is increasing as the electricity grid decarbonises. In a world where we need to reduce our carbon footprint from around 10 tons CO2 per person per year to nothing as quickly as possible, that makes a difference.