UK Heat Pump Capacity by House Size 2026
The kW sizing question answered for every UK home type — 1-bed flat to 5-bed detached, new build to Victorian solid wall. Real charts, real formulas, and the four sizing mistakes installers make.
The sizing formula in one line
For a quick estimate: kW heat pump = floor area in m² × heat loss factor W/m² ÷ 1,000.
Heat loss factor depends on construction type, insulation level, and exposure. Here are the UK averages from BRE Domestic Energy Fact File 2024:
| Property era / construction | Heat loss factor | Notes |
|---|---|---|
| New build 2010+ (Part L modern) | 45 W/m² | U-values around 0.18 walls, 0.13 roof; MVHR common |
| 1980-2010 cavity wall, well insulated | 55 W/m² | Filled cavity, double glazing, loft 270mm |
| 1950-1980 cavity wall, basic insulation | 70 W/m² | Cavity unfilled or partial, single → double glazing retrofit |
| Pre-1950 solid wall, no insulation | 85 W/m² | 9-inch brick or stone, draughty windows |
| Victorian / Edwardian / Georgian (often listed) | 95 W/m² | Solid wall, high ceilings, original windows |
| Stone-built (Cotswold, Pennine, Highland) | 100-110 W/m² | Higher thermal mass but worse U-values; exposed locations |
kW sizing by house type — the full chart
The table below uses the formula above with typical UK floor areas. For your specific home, replace the floor area with your actual measurement.
| Property type | Floor area | New build | 1980-2010 | 1950-1980 | Pre-1950 | Victorian |
|---|---|---|---|---|---|---|
| 1-bed flat | 40-55 m² | 2-3 kW | 3-4 kW | 3-5 kW | 4-5 kW | 4-6 kW |
| 2-bed flat / mid-terrace | 55-75 m² | 3-4 kW | 4-5 kW | 4-6 kW | 5-7 kW | 6-8 kW |
| 2-bed end-terrace | 70-85 m² | 3-4 kW | 4-5 kW | 5-7 kW | 6-8 kW | 7-9 kW |
| 3-bed semi | 85-110 m² | 4-5 kW | 5-7 kW | 6-8 kW | 7-10 kW | 8-11 kW |
| 3-bed terrace | 90-115 m² | 4-5 kW | 5-7 kW | 7-9 kW | 8-11 kW | 9-13 kW |
| 3-bed detached | 100-130 m² | 5-6 kW | 6-8 kW | 7-10 kW | 9-12 kW | 10-13 kW |
| 4-bed semi | 110-140 m² | 5-7 kW | 7-9 kW | 8-11 kW | 10-13 kW | 11-14 kW |
| 4-bed detached | 140-180 m² | 7-9 kW | 8-11 kW | 10-14 kW | 12-16 kW | 14-18 kW |
| 5-bed detached | 180-240 m² | 9-11 kW | 10-14 kW | 13-17 kW | 16-20 kW | 17-22 kW |
Notes: these are steady-state design kW, not peak. Modern inverter heat pumps modulate down to 25-30% of rated output, so a 10 kW unit running at 4 kW most of the year is normal and efficient.
Worked examples
Example 1: 1980s 3-bed semi in Reading
- Floor area: 100 m²
- Construction: cavity wall (filled), double glazing, 270mm loft insulation
- Heat loss factor: 55 W/m²
- Calculation: 100 × 55 ÷ 1,000 = 5.5 kW → round up to 6 kW
Choose a 6 kW Mitsubishi Ecodan R290, Vaillant aroTHERM Plus 7 kW (next available size up), or Grant Aerona 290 6 kW. All would work.
Example 2: Victorian 3-bed terrace in Cambridge
- Floor area: 130 m²
- Construction: solid 9-inch brick, single-glazed sash windows, 100mm loft insulation
- Heat loss factor: 95 W/m²
- Calculation: 130 × 95 ÷ 1,000 = 12.4 kW → choose 12-13 kW
This will need a Mitsubishi Ecodan 14 kW or Vaillant aroTHERM Plus 12 kW. Strongly consider insulation upgrades first — adding 270mm loft insulation and secondary glazing could drop the requirement to ~10 kW.
Example 3: New build 4-bed detached in Milton Keynes
- Floor area: 160 m²
- Construction: Future Homes Standard, U-value 0.18, MVHR fitted
- Heat loss factor: 45 W/m²
- Calculation: 160 × 45 ÷ 1,000 = 7.2 kW → choose 7-8 kW
Developers typically pre-fit a 7 or 8 kW unit. Verify against the SAP calc in your handover pack.
Hot water cylinder sizing — separate decision
Cylinder size isn't determined by kW — it's determined by household hot water demand. Rules of thumb:
| Household | Cylinder size |
|---|---|
| 1-2 person | 150-180 L |
| 2-3 person, 1 bathroom | 200-210 L |
| 3-4 person, 1-2 bathrooms | 250 L |
| 4-5 person, 2 bathrooms | 300 L |
| 5+ person or 3+ bathrooms | 350-400 L (or twin cylinder) |
Heat pump cylinders need a larger coil surface area than gas-boiler cylinders to allow heat exchange at lower flow temperatures. Always specify a "heat pump ready" cylinder (Megaflo HP, Joule HP, Telford Tristar HP).
The four sizing mistakes installers make
- Like-for-like with the old gas boiler. Most UK gas boilers are 24-30 kW — vastly oversized for heat output (they were sized for fast hot water on demand). A heat pump replacing a 28 kW boiler typically needs 6-10 kW, not 28 kW. Don't accept "your old boiler was 28 kW so you need a 28 kW heat pump".
- Adding a margin "for safety". Oversizing causes short-cycling and tanks SCOP. A correctly sized pump with weather compensation runs steady-state and is more reliable.
- Ignoring insulation upgrades. A homeowner planning loft insulation should size the heat pump for the post-insulation house, not the current state.
- Not doing a room-by-room calc. Whole-house averaging hides hot spots (large lounges, conservatories) that need oversized radiators. Insist on room-by-room.
What "right-sized" looks like in practice
A well-sized installation:
- Runs steady-state for 70-80% of winter hours (no short-cycling)
- Hits its design output at -2°C without resorting to immersion heater
- Modulates down to 25-30% of rated output in mild weather
- Achieves design SCOP within 0.2 (e.g., 3.4 designed, 3.2-3.6 actual)
If your install fails any of these, the sizing was wrong or commissioning was incomplete.
When to choose the next size up
The formula gives you a target. Round up to the next available size IF:
- You have an exposed location (coast, hilltop, North Scotland)
- Your home has significant air leakage (pre-1970, no draught-proofing)
- You plan large extensions soon
- You have unusually high hot water demand (5+ occupants, 3+ bathrooms)
- You need 21°C+ indoor setpoint (elderly, infants, medical needs)
Heat loss calc vs the rule of thumb
This article gives rule-of-thumb sizing. Your installer MUST produce a room-by-room heat loss calculation per BS EN 12831 before quoting. The calc accounts for:
- Specific U-values for your walls, roof, floor, windows
- Air change rate
- Internal heat gains (occupants, appliances)
- Solar gains
- Room-specific setpoints (bathroom 22°C, lounge 21°C, bedroom 18°C)
- Your specific design outdoor temperature (-2°C London, -5°C Edinburgh, -7°C Highlands)
The room-by-room calc is usually accurate within ±10%. The rule-of-thumb above is within ±20-30%. Use it for sanity-checking quotes; don't use it to override an installer's calc.