Free Eco Home Calculators

Heat Pump Sizing Calculator

1 min read Last updated 20 May 2026
Quick answer A typical UK 3-bed semi needs around 7-9 kW of heat output for an air source heat pump. Installed cost is roughly £12,000-£14,000 before the £7,500 BUS grant, leaving a net cost of £4,500-£6,500. Running cost in 2026 is broadly comparable to a gas boiler, with savings against oil, electric or LPG.

The Heat Pump Sizing Calculator works out the heat output (in kW) your home actually needs, recommends a sensible system size, estimates running cost at 2026 electricity rates, compares it against your current heating fuel, and applies the £7,500 Boiler Upgrade Scheme (BUS) grant to give a net install cost and payback period.

Heat Pump Sizing Calculator

Determine the right heat pump size for your property and estimate running costs.

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See 20 real-world examples

Each link below runs this calculator with real UK city and property inputs and publishes the exact computed savings, payback time, install cost and CO2 figures.

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How this calculator works

Heat demand uses the watts-per-square-metre method: 35-120 W/m² depending on insulation level (excellent → poor), modified by property type (detached loses more than terraced). We add a hot-water allowance of 1.5-4.5 kW. Annual electricity demand is heat demand × 2,000 full-load equivalent hours / COP 3.0. Running cost uses the 2026 Ofgem cap (24.5p per kWh). The BUS grant is fixed at £7,500 in England and Wales (up to £9,000 in Scotland via Home Energy Scotland).

Sources & references

Frequently asked questions

Are heat pumps cheaper to run than gas in 2026?

It depends. Electricity is about 3.4× the per-kWh price of gas, so a heat pump needs a SCOP of at least 3.4 to break even purely on running cost. Most well-installed UK systems achieve 3.0-3.5. Total cost (including standing charges if you remove your gas connection) is usually within £100-£300 a year of a modern gas boiler.

How big a heat pump do I need?

Multiply your floor area in m² by 35 (for excellent insulation) up to 120 (for poor insulation), divide by 1,000 to get kW, then add 1.5-4.5 kW for hot water. A 100m² semi with average insulation typically needs 8-9 kW.

Does the £7,500 BUS grant cover the whole cost?

No — the grant covers about 50-60% of a typical £12,000-£14,000 install. Some Scottish homes can stack the BUS-equivalent up to £9,000 via Home Energy Scotland plus an interest-free loan covering the remainder.

Will my radiators need replacing?

Sometimes. Heat pumps run at lower flow temperatures (45-50°C vs 70-80°C for gas), so radiators may need to be larger to deliver the same heat output. A good MCS installer surveys this room-by-room before quoting.

How disruptive is the install?

A standard ASHP swap-in (no radiator changes) takes 2-3 days. With radiator upgrades and a hot water cylinder install, allow 4-6 days. Most installations don't require digging up the garden — the outdoor unit sits on a small concrete pad or wall bracket.

Related guides

Tool: Heat Pump Sizing Calculator. Last reviewed: . Figures based on Ofgem price cap, gov.uk grant guidance, and Energy Saving Trust advice. Verify scheme eligibility with your installer before commissioning work.