Free tool

What size heat pump do I need?

Heat pumps are rated in kW (and sometimes BTU). Too small and it won't keep up on cold days; too big and it short-cycles, wasting energy. Enter your room or home size and we'll estimate the output you need.

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How the estimate works

We start from roughly 400 BTU per m² of floor area — a widely used rule of thumb for UK rooms with standard ceilings — then adjust for insulation quality, the number of people (each person adds body heat), and whether it's a kitchen (appliances add a lot). For an exact figure, a MCS-certified installer will do a full heat-loss survey, which is the gold standard before installing a heat pump.

What size heat pump for a whole house?

For whole-home heat pumps, the sizing depends on your property's heat loss (insulation, glazing, number of rooms). As a rough guide:

• 1-2 bed flat / terraced (up to 80 m²): 3–5 kW heat pump
• 2-3 bed semi-detached (80–120 m²): 5–8 kW heat pump
• 3-4 bed detached (120–180 m²): 8–12 kW heat pump
• Large detached (180+ m²): 12–16 kW or a cascade system

These figures assume average insulation. A well-insulated home needs less; a Victorian property with solid walls may need more. Your installer will calculate the exact size during a site survey.

Room-by-room guide

• Small bedroom / office (up to 15 m²): ~5,000–7,000 BTU (1.5–2 kW)
• Average bedroom / living room (15–30 m²): ~7,000–12,000 BTU (2–3.5 kW)
• Large living room / open plan (30–45 m²): ~12,000–18,000 BTU (3.5–5.3 kW)

Does a heat pump's size affect the Boiler Upgrade Scheme grant?

Yes — the Boiler Upgrade Scheme gives £7,500 regardless of heat pump size, but the total installation cost will vary. A larger heat pump costs more to install, so your out-of-pocket cost after the grant will be higher. Getting the right size is important — oversizing wastes money and reduces efficiency.

This tool gives a ballpark for guidance only and isn't a substitute for a professional heat-loss survey from an MCS-certified installer.