Domestic air source heat pump running costs depend on three numbers: the heat your household needs, the system’s seasonal efficiency and the electricity price you pay. There is no reliable annual bill for every home. These calculations show how to estimate your own costs and compare heating options on equal terms.
Use the unit rate, not the headline price cap
For 1 October to 31 December 2026, Ofgem’s Great Britain average Direct Debit price-cap rates are 26.32p/kWh for electricity and 7.97p/kWh for gas. Regional rates and fixed or specialist tariffs differ. The electricity rate reflects the temporary VAT treatment for that period. Use your bill’s rate for a personal estimate. See Ofgem’s dated unit-rate tables.
The headline annual cap represents a particular consumption assumption. It is not the maximum bill a heat pump household can receive. More electricity use means a larger bill, even when the unit rate is correctly capped.
Convert useful heat into electricity
A seasonal performance factor, or SPF, compares delivered heat with electricity consumed over the measurement period. At SPF 3, the system supplies three heat units for each electricity unit counted within that boundary. A best-case laboratory COP should not replace a whole-year estimate.
Calculate annual useful heat ÷ SPF × electricity price. Confirm whether circulating pumps, backup heaters and hot water are included. If excluded, add them separately; otherwise a precise-looking forecast can understate consumption.
A transparent annual cost example
Assume a home needs 12,000kWh of useful heat annually, covered by the chosen efficiency estimate. At 26.32p/kWh, SPF 3 requires 4,000kWh of electricity and costs £1,052.80. SPF 3.5 costs about £902.40; SPF 4 costs £789.60.
These are our calculations with illustrative demand and the October to December 2026 rate held constant for a full year. They are not measured household averages, tariff forecasts or promised savings. Colder weather, warmer rooms or additional hot water can change the result.
Compare the boiler’s useful output
Purchased boiler fuel is not identical to useful delivered heat. Illustratively, 12,000kWh of useful heat from a boiler averaging 90% efficiency requires around 13,333kWh of gas. At 7.97p/kWh that costs about £1,062.67, excluding standing charges and servicing.
On those assumptions, a heat pump needs SPF roughly 2.97 to match the boiler’s fuel-only cost. Different tariffs or boiler efficiency change the threshold. This is why every heat pump cannot be assumed cheaper than gas.
Keep standing charges and maintenance separate
Your home already pays an electricity standing charge, so allocating the whole charge to the heat pump can distort a before-and-after comparison. Removing gas may avoid its standing charge only when no gas appliances remain and the supply arrangements actually change.
Keep routine servicing and finance payments separate from electricity consumption. They matter to the household budget, but combining them obscures whether the heating equipment is using the expected energy.
Lower consumption with informed adjustments
Begin with the weather compensation and schedule agreed at commissioning. Forcing higher flow temperatures or unnecessary resistance backup heating can undermine projected efficiency. Record meter readings, room temperatures and settings before changing several things at once.
For a household in Horsham, ask Air Source Heat Pumps West Sussex for the useful heat demand, efficiency boundary and tariff behind an installation estimate. Those details explain more than a monthly figure borrowed from another house.
Make tariff comparisons realistic
A specialist heat pump tariff can have several time bands. Apply each rate to a plausible consumption profile rather than calculating the entire year at the cheapest overnight rate. A home requiring heat throughout the day will not necessarily move all demand into a short discounted window.
Consider the rest of the electricity account too. A tariff that benefits heating could increase the cost of cooking or other daytime use, and export terms may change the solar calculation. Compare the whole household bill before switching. Keep a copy of the rates and dates used so that a later price change can be distinguished from a change in equipment performance.
Investigate unexpectedly high bills
Start by checking meter readings, billing dates and whether the supplier estimated consumption. Compare similar periods with notes about weather and occupancy. More guests, a higher thermostat setting or a long cold spell can increase demand without indicating a fault.
If electricity remains unexpectedly high, ask a specialist to review flow temperatures, schedules and auxiliary heating operation. Avoid reducing stored-water safety temperatures or disabling safeguards to chase a lower bill. Evidence about settings and consumption gives the investigation a useful starting point.
Summary
A meaningful estimate shows its workings. Use your property’s heat requirement, realistic seasonal efficiency and your actual unit price, then separate standing charges and maintenance. Our home installation service can help you evaluate a proposal with its assumptions clearly explained.
