Methodology and sources

Every number the calculator shows comes from the model below. Nothing is hidden and nothing is made up. Last updated 2 July 2026.

Heat demand

We estimate your home’s annual heat demand (in kWh) from three inputs: a base figure per bedroom count, a factor for the property type (a flat needs less heat than a detached house of the same size), and an insulation factor. The insulation factors are poor (epc e-g) 1.3, fair (epc d) 1.1, good (epc c) 1, excellent (epc a-b) 0.8. If you already know your annual heating kWh from your bills or EPC, you can type it in directly and skip the estimate.

Running costs

Heat pump cost per year = heat demand ÷ SCOP × electricity unit price. SCOP defaults to 3.5, a mid-range seasonal efficiency for a modern air source heat pump; you can set it between 2.5 and 4.5. Gas boiler cost = heat demand ÷ 0.9 (boiler efficiency) × gas unit price, plus the gas standing charge where you choose to drop gas entirely.

Default prices are the Ofgem price cap for July to September 2026: electricity 26.11p/kWh, gas 7.33p/kWh, with standing charges of 57.19p/day (electricity) and 29.04p/day (gas). Regional prices use the cap for each of the 14 supply regions. Source: Ofgem energy price cap.

The heat pump tariff scenario uses ~15p/kWh, based on the dedicated time-of-use tariffs available in 2026: Octopus Cosy at 14.53p in its cheap windows, Scottish Power Heat Pump Saver and Good Energy Heat Pump at about 14p, and EDF Heat Pump Tracker at about 14.9p. It assumes most heating runs inside the cheap windows. Source: Sunsave heat pump tariff comparison.

Install cost, grant and payback

System size is estimated from heat demand, and a typical installed price is suggested for that size; you can overwrite it with your own quote. The £7,500 grant is the Boiler Upgrade Scheme (England and Wales). Payback = (install cost − grant) ÷ annual saving. If there is no saving at your inputs, no payback is shown.

CO2

Carbon savings compare your current fuel’s emissions factor with electricity’s grid average, using UK government conversion factors.

What this is not

These are estimates for comparison, not quotes or financial advice. Real costs depend on your home’s survey, the system installed, how you run it, and your tariff. Always get multiple quotes from MCS-certified installers.

Questions or corrections: contact us.