What a normal bill looks like
Start with the sum, so you know what “high” means. A heat pump’s electricity use is the heat your home needs divided by the system’s seasonal efficiency. For an illustrative home that needs 12,000 kWh of heat a year, a heat pump running at an efficiency of 3 uses about 4,000 kWh of electricity. At the Ofgem cap for 1 October to 31 December 2026 (26.32p per kWh), that is about £1,053 a year. You already paid the electricity standing charge, about £200 a year at the cap, before the heat pump arrived, so it is not a new cost.
Efficiency moves the number more than anything else. The same home at an efficiency of 2.5 would pay about £1,263, and at 2.0 about £1,579. So a system that is working, but poorly, can cost a few hundred pounds a year more than the design promised, without anything being broken.
| Seasonal efficiency | Electricity used | Cost a year |
|---|---|---|
| 3 | 4,000 kWh | £1,053 |
| 2.5 | 4,800 kWh | £1,263 |
| 2.0 | 6,000 kWh | £1,579 |
Cause 1: the flow temperature is too high
A heat pump is most efficient when it heats water to a low temperature and runs steadily. The Energy Saving Trust says radiator flow temperatures with a heat pump are often between 35 and 45°C, against about 75°C for a typical boiler. The Electrification of Heat trial measured how much that matters: the median heat pump had an efficiency of 3.5 at a 35°C flow, 2.38 at 55°C and 2.02 above 65°C. The average operating flow temperature across the trial’s air source systems was 39.7°C.
If your system was set up hot to keep rooms warm with small radiators, or someone has turned the flow temperature up, the bill follows. Ask your installer what flow temperature the design assumed, and whether weather compensation is switched on so the temperature falls on mild days. The controls and weather compensation guide explains the settings.
Cause 2: the system is below its designed efficiency
This is common. In the trial, 91.9% of homes measured a lower efficiency in use than the system’s design figure, with a median shortfall of 17.9%. The median air source system measured 2.78 over a year on the whole-system measure. A gap does not mean a fault, but a large one is worth questioning: undersized radiators, a poorly balanced system, controls set like a boiler, or a heat-loss estimate that did not match the house.
This is where the quote matters. A proper design starts with a room-by-room heat-loss calculation and states the expected efficiency and flow temperature in writing. Those line items are covered in heat pump installation cost, and they are what you go back to if the bill does not match the promise. If the installer will not look at it, the complaints and consumer codes guide sets out the route.
Cause 3: the tariff
On a flat default tariff at the cap you pay 26.32p for every kWh, whenever you use it. Each 1p off the average price you pay is worth about £40 a year on 4,000 kWh. Some suppliers sell tariffs built for heat pumps with cheaper hours: one example, Cosy Octopus, said on 8 October 2026 that its cheap periods were 51% cheaper than its day rate and its peak was 50% above it. The saving depends on whether your system can run in the cheap hours, so compare on your own use. The heat pump tariffs page does the arithmetic.
Cause 4: hot water, boost and back-up heaters
Hot water is heated in a cylinder, and the Energy Saving Trust notes the cylinder needs heating above 60°C from time to time for Legionella protection. If that cycle runs too often, or an immersion heater is doing the work instead of the heat pump, use rises sharply, because an immersion heater turns one unit of electricity into one unit of heat. Check the schedule, and check whether any electric back-up or boost is switched on that you did not ask for.
What to check, in order
- Compare like with like. Add your old gas or oil spend to your old electricity bill, and compare that total with the new electricity bill across the same months.
- Read the flow temperature on the controller, and ask the installer what the design assumed.
- Check weather compensation is on, and that there are no aggressive boost or set-back schedules.
- Look at the hot water schedule and any immersion or back-up heater.
- Ask for the commissioning figures: the designed efficiency, heat loss and flow temperature, so you can compare them with what the system is doing.
- Review the tariff once the system is running as designed.