What Size Heat Pump Do I Need? A Plain-English Guide
Heat pump sizing is the most common question installers face. Too small and it won't cope on cold days. Too large and it short-cycles. Here is what determines the right kW for your home.
Sizing a heat pump correctly is the single most important factor in whether your installation performs well or is a disappointment. Too small and the system struggles on cold days, running constantly and raising electricity bills. Too large and it short-cycles - switching on and off too frequently - which reduces efficiency, increases wear, and creates uneven heating.
The right size is determined by a heat loss calculation. This is a formal assessment of how quickly your home loses heat on the coldest design day - typically -3°C for most of England and Wales, colder for Scotland and elevated sites. The calculation accounts for wall construction and insulation, floor area, ceiling height, window quality, and the number of exposed external walls.
UK standard BS EN 12831 governs professional heat loss surveys. Any MCS-certified installer must carry one out before specifying your system - it is a condition of the BUS grant. Be wary of any installer who quotes without doing this survey.
As a rough guide: a typical 3-bedroom semi-detached house with average insulation needs roughly 7-9 kW. A 4-bedroom detached with poor insulation might need 12-14 kW. A 2-bedroom terrace with good insulation could be fine with 5-6 kW. These are starting points only.
One of the most common mistakes is treating the old boiler's kW rating as a guide. Gas boilers run at high flow temperatures (70-80°C) and need high outputs. Heat pumps run at lower temperatures (35-55°C) and are sized much smaller for the same job. A home heated by a 28 kW boiler will typically need a 7-10 kW heat pump.
Use our heat pump size calculator for a property-specific estimate, then get an MCS installer to confirm with a proper survey before any equipment is ordered.
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Disclaimer: Prices and specifications correct as of April 2026. Always get a professional heat loss assessment before purchasing. We are not installers and do not provide heating advice.