What Size Heat Pump Do I Need?

Sizing a heat pump correctly is critical. Too small and it won't heat your home on cold days. Too large and it short-cycles, reducing efficiency and lifespan. This calculator gives you a ballpark figure — a full heat loss survey from an MCS installer is the definitive answer.

Property type

Number of bedrooms

Insulation level

Property built

How heat pump sizing works

A heat pump must be sized to your property's peak heat loss — the rate at which heat escapes on the coldest design day (typically -3°C in most of England and Wales).

The key factors are: floor area, number of external walls and their construction, window quality, roof insulation, and property age. A detached house loses heat from all four sides; a mid-terrace from only two.

UK standard BS EN 12831 governs professional heat loss calculations. Any MCS-certified installer must complete this survey before specifying your system — it is a requirement of the BUS grant.

Common sizing mistakes

Oversizing 'just in case'

A heat pump that is too large for the property will short-cycle — turning on and off frequently. This reduces efficiency, increases wear, and produces less even heating than a correctly sized unit.

Copying the old boiler's kW rating

Gas boilers are typically 24–35 kW because they run at high flow temperatures (70–80°C). Heat pumps run at 35–55°C and need a much lower kW rating for the same job. Your installer should not just match the boiler size.

Ignoring insulation improvements

Improving loft insulation (£300–600) or cavity wall insulation (£600–1,500) before installation can reduce the required heat pump size by 2–4 kW, potentially saving £500–1,000 on the unit and improving running costs for 20+ years.