Answer first
For an illustrative home with an 8kW design heat loss, averaging half that load over 24 hours would mean about 96kWh of useful heat. At a cold-day COP of 2.5, the heat pump would use roughly 38.4kWh. At 26.11p/kWh that is about £10.03 for the day, before any standing charge.
That is deliberately an example, not a typical-home claim. A smaller, better-insulated home may need far less heat; a large exposed home may need more. Weather, wind, hot water, defrost and backup heat also matter.
Mean whole-system COPH4 on the monitored project’s coldest day
View sourceMean outdoor temperature for that coldest-day analysis
View sourceA whole-day estimate must account for changing load, not assume full output all day
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Rated capacity is not daily consumption
An 8kW heat pump does not normally draw 8kW of electricity continuously. The rating describes heat output at stated conditions. On most hours the building load is below its coldest design point, and a well-controlled inverter-driven unit modulates its output.
Multiplying the model rating by 24 therefore exaggerates consumption. Start with the building heat needed across the actual day, add hot water and then divide by the efficiency achieved while delivering it.
A worked cold-day example
Assume a surveyed design heat loss of 8kW at the local design temperature. If conditions and internal gains mean the home averages 4kW across one cold day, it needs 4 × 24 = 96kWh of space heat. At COP 2.5, that requires 38.4kWh of electricity.
At 26.11p/kWh the space-heating cost is £10.03. Add whatever electricity the system uses for hot water, circulation and any deliberate backup heat. Changing the average load to 3kW reduces the example to 72kWh of heat and 28.8kWh of electricity at the same COP.
Keep in mind: Use the unit rate on your tariff. Ofgem publishes averages, but the household’s actual rate and time-of-use periods decide the bill.
Cold weather reduces efficiency, but not to one fixed number
The Electrification of Heat project found a mean COPH4 of 2.44 on the coldest analysed day, when the mean temperature was −0.4°C. That shows useful heat was still being delivered at more than twice the electrical input at that system boundary, on average across the sample.
Individual results varied. Required water temperature, defrost cycles, unit selection, emitter capacity and system controls all influence cold-weather performance. A quote should show capacity and efficiency at relevant design conditions, not only a mild-weather figure.
Why one expensive day does not predict the year
Annual cost is the sum of mild, average and cold days. A home may have several high-consumption days during a cold spell but spend much of the heating season at lower load and better efficiency. Seasonal performance is more useful for an annual budget.
Compare equivalent comfort as well. A household that heats more rooms for longer after installation may use more useful heat even if the system is efficient. That is a lifestyle and comfort change, not automatically a fault.
The questions to ask before accepting a winter-cost claim
A defensible estimate should let you identify the building load, the weather assumption and the system performance assumption. If one of those is missing, the final pound figure is difficult to test.
- What is the calculated design heat loss?
- What outdoor temperature is used for the location?
- What capacity and COP does the selected unit provide there?
- What flow temperature is assumed?
- How are hot water, defrost and backup heat treated?
- Which tariff and date support the price calculation?
What to read next
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Questions people also ask
Do heat pumps stop working below freezing?+
No. Correctly selected air-source heat pumps operate below freezing, but capacity and efficiency vary with outdoor and water temperatures and must be checked in the design data.
Is defrost wasted electricity?+
Defrost is a normal part of air-source operation in certain cold, damp conditions. Excessive or problematic cycling should be investigated, but the presence of defrost is not itself a fault.
Should I turn the heat pump off during an expensive cold spell?+
Usually not simply because it is cold. Allowing the home to cool deeply can require higher output and temperatures later. Use the commissioned controls and seek advice before changing the strategy.
Evidence used for this answer
Figures are explained in context and links go to the independent source. Prices, tariffs, scheme rules and standards can change, so check the source again before making a financial or technical decision.
- Energy Systems Catapult: heat-pump performance analysisMeasured seasonal performance and cold-weather operation from the Electrification of Heat demonstration project.
- Ofgem: energy price cap unit ratesPublished electricity and gas unit-rate benchmarks. Rates vary by region, payment method, tariff and price-cap period.
- MCS heat-pump design standardTechnical requirements for heat loss, equipment selection, domestic hot water and system design.
- Energy Saving Trust: the most efficient way to run a heat pumpGuidance on weather compensation, longer run periods, setbacks, radiators, hot water and smart energy use.