Heat pump size calculator: a first estimate of the kW you need
This heat pump size calculator estimates how many kilowatts of heat your home loses on a cold day, which is the output a heat pump has to deliver. Enter the heated floor area by square metre or square foot, pick an insulation level and your location, and it gives the design heat loss in kW and BTU/h, the nearest common heat pump size, how much bigger your radiators need to be at a low flow temperature, and a yearly running cost from a SCOP figure.
It is built for UK homes — the outside design temperatures and room temperatures come from the MCS heat pump design standard — but the ft² and °F option works for anywhere. It is a starting point for a conversation with an installer, not a replacement for a proper heat-loss survey.
How to use the Heat Pump Size Calculator
- Choose Whole house for a quick estimate or Room by room to see each room’s heat loss and radiator size.
- Pick the insulation level closest to your home, or type your own W/m² figure if you have one from a survey.
- Enter the heated floor area of all storeys (or each room’s area and type).
- Choose your nearest UK location and design condition, or type an outside design temperature for elsewhere.
- Select the flow temperature you are aiming for, then add your annual heat demand, a SCOP and your electricity price for the running cost.
Heat Pump Size Calculator features
UK design temperatures
Outside temperatures for eight UK cities at 99 % and 99.6 % of hours, and internal temperatures per room type, from MCS MIS 3005-D Issue 2.0.
Square metres or square feet
Switch between m² and °C or ft² and °F; results are shown in kW, BTU/h and tons.
Room by room
Each room uses its own design temperature — 22 °C bathrooms, 18 °C bedrooms — and gets a radiator output at ΔT50 to compare with existing radiators.
Flow temperature and radiators
Shows how much larger radiators must be at 35–55 °C flow than their catalogue rating.
Running cost
Annual heat demand ÷ SCOP × your electricity price.
How the heat pump size is estimated
Heat loss rises in proportion to the difference between inside and outside temperature. The calculator uses a heat loss intensity — watts per square metre of floor at 21 °C inside and −3 °C outside, a 24 K difference — and scales it to your temperatures: heat loss = area × intensity × (inside − outside) ÷ 24.
Worked example: a 90 m² partly insulated house (about 70 W/m²) in London, designed for −1.7 °C outside and 21 °C inside, loses 90 × 70 × 22.7 ÷ 24 = 5,959 W, about 6.0 kW. Using the 99.6 % temperature of −3.0 °C it is 6.3 kW. A heat pump is then chosen whose output at that outside temperature and your flow temperature covers the loss — nameplate figures are often quoted at 7 °C outside and 35 °C flow, which flatters them.
| Insulation level (rule of thumb) | W/m² at ΔT 24 K | 90 m² home |
|---|---|---|
| Passive-house level | 15 | 1.4 kW |
| New build to recent regulations | 30 | 2.7 kW |
| Well insulated | 50 | 4.5 kW |
| Partly insulated, 1960s–1990s | 70 | 6.3 kW |
| Poorly insulated, solid walls | 100 | 9.0 kW |
| Very poor, single glazed | 130 | 11.7 kW |
What the MCS standard requires
- MIS 3005-D (Issue 2.0, 2 December 2024) asks for a heat-load calculation to BS EN 12831-1:2017 using internal temperatures no lower than its Table 1 (living and dining rooms 21 °C, bedrooms, halls and kitchens 18 °C, bathrooms 22 °C) and the outside temperatures of its Table 2 (column A, or column B with no uplift for intermittent heating).
- The heat pump must supply at least 100 % of the calculated heat load at the design flow temperature without the backup electric heater (hybrid systems: at least 55 % at 55 °C flow).
- High-temperature heat pumps should be avoided unless a flow above 55 °C is needed, and any design above 55 °C must be offered alongside one at 55 °C or lower.
- The installer gives you the specific heat loss (W/m²), the design flow temperature and the SCOP from the MCS database.
Radiators and flow temperature
Radiator catalogues quote output at ΔT50 — mean water 50 °C above the room. At a 45 °C flow (about 42.5 °C mean) a 21 °C room sees only ΔT 21.5, and output falls to (21.5 ÷ 50)1.3 ≈ 33 % of the rating, so the radiator needs to be about three times larger. Lower flow temperatures raise the SCOP and cut running costs, which is why heat pump installs often replace a few radiators. Compare a gas system with the boiler size calculator, check your current gas use with the gas units to kWh calculator, and see the CO₂ effect in the carbon footprint calculator.
Heat Pump Size Calculator FAQ
What size heat pump do I need for my house?
Most UK homes need between 4 and 12 kW. The heat pump size calculator multiplies floor area by a heat loss intensity for your insulation level and scales it to your inside and outside design temperatures — a 90 m² partly insulated house in London needs about 6 kW.
How do I size a heat pump by square metre or square foot?
Multiply the heated floor area by a heat loss figure per m² (or per ft²) for the insulation level: roughly 30 W/m² for a new build, 50 for a well-insulated home and 70–100 for older houses at −3 °C outside. These are rules of thumb; a survey replaces them with measured fabric losses.
Is a bigger heat pump better?
No. An oversized heat pump costs more and cycles on and off in mild weather, which lowers efficiency. MCS requires it to meet 100 % of the calculated heat load, not a large margin above it.
What is SCOP?
Seasonal coefficient of performance: heat delivered over a year divided by electricity used. A SCOP of 3.2 means 12,000 kWh of heat needs about 3,750 kWh of electricity. It is higher at lower flow temperatures.
Do I need bigger radiators for a heat pump?
Often in some rooms. At 45 °C flow a radiator gives about a third of its ΔT50 rating, so the room-by-room mode shows the ΔT50 output each room’s radiator would need.
Can I use this instead of an MCS heat loss survey?
No. It is a rough estimate for budgeting. MCS installations must be designed from a room-by-room heat-loss calculation, and grants such as the Boiler Upgrade Scheme require an MCS-certified installer.