Methodology and sources

Every number the calculator shows comes from the model below. Nothing is hidden and nothing is made up. Last updated 14 September 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 1 October to 31 December 2026: electricity 26.32p/kWh, gas 7.97p/kWh, with standing charges of 54.83p/day (electricity) and 29.68p/day (gas), paying by Direct Debit. Regional prices use the cap for each of the 14 supply regions. Each quarter’s cap is stored with its start date and the site switches to it on that day. Source: Ofgem energy price cap unit rates and standing charges.

There is no VAT on electricity from 1 October 2026 to 31 March 2027, according to Ofgem; gas still carries 5% VAT.

Heating oil (7p/kWh) and LPG (9.5p/kWh) are not covered by the price cap. These defaults are estimates, not sourced live prices, so enter the price from your latest delivery for a real comparison.

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. The tariff rates were checked in June 2026, when electricity carried 5% VAT, so the tariff scenario may slightly overstate current tariff costs until the rates are re-checked.

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). Until March 2027 the calculator applies £9,000 instead when the current fuel is oil or LPG, because homes heated by oil or LPG with no mains gas connection get an extra £1,500; it returns to £7,500 after 31 March 2027. In Scotland the Boiler Upgrade Scheme does not apply: Home Energy Scotland offers grant funding of up to £7,500 for a heat pump, an extra £1,500 for rural and island homes, and an optional interest-free loan of up to £7,500, and the Scottish pages use the £7,500 grant. 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.