Heat Pump Size Calculator
Work out the correct kW heat pump size for your UK home — by floor area, age, and insulation level. Based on the MCS Heat Emitter Guide and real installer sizing data.
Your property
This is a guide. Your MCS installer will run a room-by-room heat loss calculation that may differ by ±20%.
How heat pump sizing works
Heat pump sizing isn't guesswork — there's a clear three-step calculation defined in the MCS Heat Emitter Guide. Here's the simplified version.
Heat loss per m²
Based on property age & insulation: 30 W (modern) to 110 W (solid-wall).
Total heat demand
Area × W/m² × ceiling factor × climate factor = kW needed at design temp.
Apply 1.5× design margin
Pump must hit demand at coldest expected day (-3°C in most UK regions).
Round to next available size
Heat pumps come in 5, 6, 8, 9, 11, 14, 16 kW — pick the nearest sensible up.
Cross-check with annual demand
UK annual heating demand: ~50–240 kWh/m² depending on insulation.
The cost of getting heat pump sizing wrong
Oversizing is by far the most common installer mistake in the UK heat pump industry. Here's what happens with a heat pump that's too big:
- Short cycling — the compressor turns on, hits target temp quickly, turns off, then needs to start again 15 minutes later. Compressor starts are the wearing event; double the starts halves the unit's lifespan.
- SCOP collapses — heat pumps are most efficient when running steadily at part load. An oversized pump never reaches that efficient steady-state.
- Higher bills — the wasted electricity from short cycling can add 20–30% to your annual running cost.
- More noise — bigger units have bigger fans and bigger compressors.
Undersizing is rarer but also a problem — on the coldest 5 days of the year, the pump can't keep up and you're cold (or paying for an emergency electric heater).
UK heat loss factors explained
| Property type | Approx. age | Heat loss (W/m²) | Annual demand (kWh/m²) |
|---|---|---|---|
| Modern, well-insulated | Post-2010 build regs | ~30 W/m² | ~50 |
| Standard UK home | 1990–2010, cavity wall | ~55 W/m² | ~95 |
| Older home | Pre-1980, basic insulation | ~90 W/m² | ~175 |
| Solid wall / listed | Pre-1920, uninsulated | ~110 W/m² | ~240 |
These factors are derived from the MCS Heat Emitter Guide and SAP 10 data. They're rule-of-thumb only — your installer's full room-by-room survey may differ by ±20% based on window areas, exposure, and specific construction details.
Common UK home sizes & suggested pumps
| Home type | Floor area | Standard property | Solid-wall property |
|---|---|---|---|
| 1-bed flat | 40–55 m² | 4 kW | 6 kW |
| 2-bed mid-terrace | 65–80 m² | 5–6 kW | 9 kW |
| 3-bed semi | 85–110 m² | 7–9 kW | 11–14 kW |
| 4-bed detached | 125–160 m² | 10–12 kW | 14–16 kW |
| 5-bed detached | 170–230 m² | 13–16 kW | 16–22 kW |
Should I trust the installer's number?
Yes — after they've done a proper room-by-room heat loss survey. Trust their number less if:
- They quoted without doing a survey (red flag — should be free)
- Their quote is the only one you have
- They suggest significantly bigger than this calculator's result (10%+ above)
- They suggest the same size as your current gas boiler (gas boilers are routinely oversized by 2×; that doesn't mean your heat pump should be)
Always get 3 MCS quotes, compare the suggested sizes, and challenge the outlier. If two installers say 8 kW and one says 12 kW, the 12 kW is almost certainly wrong.
Frequently asked questions
What size heat pump do I need for a UK house?
- Modern well-insulated home: ~30 W/m² → 3 kW per 100 m²
- Standard 1990–2010 UK home: ~55 W/m² → 5.5 kW per 100 m²
- Pre-1980 home: ~90 W/m² → 9 kW per 100 m²
- Solid-wall / listed: ~110 W/m² → 11 kW per 100 m²