What Size Heat Pump Do I Need? UK kW Chart and Running Cost
A 100 m² home with average insulation loses about 7.0 kW on a cold day, so it needs about a 7 kW heat pump. It uses about 12,000 kWh of heat in a year, which costs about £1,040 to make with a heat pump at 26p and a SCOP of 3, and £859 from a gas boiler at 6.3p. The heat pump calculator works out the size and the running cost for your own home and prices.
Heat pump size by floor area and insulation
The heat the home loses on a cold day, with the common heat pump size in brackets:
| Floor area | Poor (100 W/m²) | Average (70 W/m²) | Good (50 W/m²) | Excellent (30 W/m²) |
|---|---|---|---|---|
| 50 m² | 5.0 kW (5 kW) | 3.5 kW (5 kW) | 2.5 kW (5 kW) | 1.5 kW (5 kW) |
| 75 m² | 7.5 kW (9 kW) | 5.3 kW (7 kW) | 3.8 kW (5 kW) | 2.3 kW (5 kW) |
| 100 m² | 10.0 kW (12 kW) | 7.0 kW (7 kW) | 5.0 kW (5 kW) | 3.0 kW (5 kW) |
| 125 m² | 12.5 kW (14 kW) | 8.8 kW (9 kW) | 6.3 kW (7 kW) | 3.8 kW (5 kW) |
| 150 m² | 15.0 kW (16 kW) | 10.5 kW (12 kW) | 7.5 kW (9 kW) | 4.5 kW (5 kW) |
| 200 m² | 20.0 kW (ask a designer) | 14.0 kW (14 kW) | 10.0 kW (12 kW) | 6.0 kW (7 kW) |
| 250 m² | 25.0 kW (ask a designer) | 17.5 kW (ask a designer) | 12.5 kW (14 kW) | 7.5 kW (9 kW) |
Running cost against gas, by SCOP
For the 100 m² home with average insulation. The heat pump cost is the electricity at 26p and at a cheap heat pump tariff of 15p, and the difference is against a gas boiler at 88% efficiency and 6.3p per kWh (a plus means it costs more):
| SCOP | Cost at 26p | Against gas | Cost at 15p | Against gas |
|---|---|---|---|---|
| 2.5 | £1,248 | £389 | £720 | -£139 |
| 3 | £1,040 | £181 | £600 | -£259 |
| 3.5 | £891 | £32 | £514 | -£345 |
| 4 | £780 | -£79 | £450 | -£409 |
The SCOP at which a heat pump costs the same as gas
Electricity price times boiler efficiency (88%) divided by the gas price:
| Electricity price | Gas 5p | Gas 6.3p | Gas 8p |
|---|---|---|---|
| 15p | 2.6 | 2.1 | 1.7 |
| 20p | 3.5 | 2.8 | 2.2 |
| 26p | 4.6 | 3.6 | 2.9 |
| 30p | 5.3 | 4.2 | 3.3 |
A SCOP at or above the figure means that the heat pump costs less to run. With electricity at four times the price of gas the break-even is about 3.6, so a typical heat pump runs for about the same as a gas boiler or a little more, and costs less on a cheap tariff, with solar panels, or at a high SCOP.
What makes a heat pump work well
- Insulation first. A home that loses less heat needs a smaller heat pump and can run it at a lower temperature.
- Low flow temperature. The lower the temperature of the water in the radiators, the higher the SCOP. That means larger radiators or underfloor heating (see the radiator size calculator).
- The right size. A heat pump that is too small cannot keep up on the coldest days, and one that is too big cycles on and off.
- A hot water cylinder. A heat pump needs a cylinder for the hot water, so there is a space and cost requirement.
- A good installer. Choose an MCS-certified company that does a room-by-room heat loss calculation, and ask about the Boiler Upgrade Scheme grant in England and Wales.
Heat pump or boiler?
A gas boiler is cheaper to fit and, at present prices, a little cheaper or similar to run. A heat pump uses less energy, produces no gas emissions at the home and can use cheaper power from solar panels or a cheap tariff. Look at the boiler size calculator to compare the size, and ask for quotes for both. These figures are estimates from typical heat use and the prices stated, not a design: an installer's survey is the proper basis.
Frequently asked questions
What size heat pump do I need for my house?
A 100 m² home with average insulation loses about 7.0 kW on a cold day, so a 7 kW heat pump. A poorly insulated home of the same size needs 10.0 kW (12 kW), and a well insulated one 5.0 kW (5 kW).
Is a heat pump cheaper to run than a gas boiler?
Not at standard prices: electricity costs about four times as much per kWh as gas, so with a SCOP of 3 the heat pump costs £181 a year more in this home. A SCOP of 3.6 or better, or a cheap heat pump tariff, makes it equal or cheaper.
How much does it cost to run a heat pump?
For a 100 m² home with average insulation, about £1,040 a year at 26p per kWh and a SCOP of 3, or £600 on a 15p tariff. A gas boiler costs about £859 at 6.3p per kWh.
What is a good SCOP for a heat pump?
Around 3 is typical and 3.5 to 4 is good. A well designed system with a low flow temperature, big radiators or underfloor heating gets the highest figures.
Do I need new radiators for a heat pump?
Often some of them. A heat pump runs at a lower flow temperature, so a radiator gives out less heat than with a boiler. See the radiator size calculator for the output to look for.
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Estimates only. Quantities depend on your materials, site and supplier. Confirm important numbers before you order, and check the Building Regulations and any planning rules for anything structural or close to a boundary.