Home › Heat pumps › Size estimator
Heat pump size (kW) estimator
A quick ballpark of the heat pump output your home might need. It is useful for sense-checking quotes, but it is no substitute for a room-by-room heat loss survey.
Defaults from our data file, last updated 2026-10-20.
Estimates only, not financial advice. Results use average UK figures and the assumptions shown. Your real costs depend on your home, tariff, region and installer quotes. Always get several quotes from certified installers and check current tariff rates before deciding.
Why heat pump size matters
A heat pump should be sized to match your home's heat loss on a cold winter day, which is the rate at which heat escapes through walls, windows, floors and roofs. An undersized unit struggles in a cold snap. An oversized one costs more upfront and switches on and off too often ("short cycling"), which wastes energy and wears the compressor. Gas boilers are commonly oversized, often 24–30 kW in homes that lose only 5–8 kW, so don't use your boiler's rating as a guide.
How this estimator works
We multiply your heated floor area by a typical peak heat loss per square metre for your insulation level, then add a small allowance (about 0.25 kW per person) for reheating the hot water cylinder. The result is rounded up to the nearest 0.5 kW. The heat loss figures are rules of thumb used across the industry for rough checks:
- About 30 W/m² for new or very well-insulated homes
- About 50 W/m² for homes with cavity wall and loft insulation and double glazing
- About 70 W/m² for average mid-20th-century homes
- About 100 W/m² for older, solid-walled or poorly insulated homes
These are editorial estimates, not measured values, and real homes vary widely. Draughts, ceiling height, glazing area and exposure to wind all make a big difference.
Worked example
A 100 m² semi-detached house with cavity and loft insulation (50 W/m²) loses roughly 5 kW at design temperature. Add 0.75 kW for three people's hot water and the ballpark is 6 kW. Typical UK air source heat pumps come in sizes such as 5, 6, 7, 8.5, 11 and 14 kW, so an installer quoting 5–7 kW for this house would be in a sensible range. If they quoted 14 kW, it's worth asking why.
What a proper survey involves
For the Boiler Upgrade Scheme, installers must be MCS-certified and follow MCS design standards, which include a room-by-room heat loss calculation. This uses measurements of each room, wall and window construction and your local design outdoor temperature. It also determines which radiators need upgrading to run efficiently at the low flow temperatures (often 35–50°C) heat pumps prefer. Ask to see this calculation. A good installer will happily walk you through it.
Reducing the size you need
- Top up loft insulation to at least 270 mm.
- Fill cavity walls if they aren't already filled.
- Draught-proof doors, windows and floorboards.
- Upgrade single glazing.
Each step reduces heat loss, which can mean a smaller, cheaper heat pump and lower running costs. Use our running cost calculator to see the effect of lower heat demand.
Using your gas bill as a cross-check
Another quick check uses your yearly gas use. Multiply it by your boiler efficiency (about 0.88) to get yearly heat demand, then divide by about 2,000 to 2,500 "full-load hours" to get a rough peak size. A home using 9,500 kWh of gas (Ofgem's current typical figure) needs about 8,400 kWh of heat, which suggests around 3.5–4 kW. A bigger or leakier home using 15,000 kWh suggests about 5.5–6.5 kW. If this and the floor-area method roughly agree, you have a sensible range to compare quotes against.
FAQs
What size heat pump does a 3-bed house need?
Many 3-bed UK homes need 5–8 kW, depending on insulation and size. Only a heat loss survey can say for sure.
Can I use my boiler size to choose a heat pump?
No. Gas boilers are usually heavily oversized, and a heat pump should match your actual heat loss.
Is a bigger heat pump better?
Not necessarily. Oversizing increases cost and can reduce efficiency through short cycling.
Do I need new radiators?
Sometimes. Heat pumps run at lower water temperatures, so some rooms may need larger radiators. Your installer's room-by-room calculation will show which.
Related calculators
Sources
- Energy Saving Trust – Air source heat pumps
- MCS – Find a certified installer
- GOV.UK – Boiler Upgrade Scheme: check if you're eligible
- Ofgem – Review of typical domestic consumption values: decision (2026 TDCVs)
Estimates only, not financial advice. Results use average UK figures and the assumptions shown. Your real costs depend on your home, tariff, region and installer quotes. Always get several quotes from certified installers and check current tariff rates before deciding.