How this calculator works
A heat pump is sized from the heat the home loses on a cold day, which we estimate from the floor area and the insulation (about 70 watts per square metre for an average home), and then rounded up to a common model. The running cost comes from the heat the home uses in a year (about 120 kWh per square metre for an average home), divided by the efficiency over the year (the SCOP: the heat made for each kWh of electricity) and multiplied by the price of electricity. A gas boiler turns about 88% of the gas into heat. The two cost the same at the SCOP equal to the electricity price times the boiler efficiency divided by the gas price.
Worked example: a 100 m² home with average insulation needs 7.0 kW, so a 7 kW heat pump, and uses 12,000 kWh of heat a year. At a SCOP of 3 that is 4,000 kWh of electricity, which costs £1,040 at 26p. The same heat from a gas boiler at 88% costs £859 at 6.3p, so the heat pump costs £181 more a year. The two cost the same at a SCOP of 3.63. With a SCOP of 3.5 the heat pump costs £891, and at 4 it costs £780 (less than gas). On a cheaper 15p heat pump tariff it costs £600. A 150 m² home needs 10.5 kW (12 kW pump), a poorly insulated 100 m² home 10.0 kW and a well insulated one 5.0 kW.
Heat pump size and running cost by floor area (average insulation, SCOP 3, 26p and 6.3p)
Compared with a gas boiler at 88% efficiency.
| Floor area | Heat load | Heat pump | Heat used | Heat pump cost | Gas boiler cost |
|---|---|---|---|---|---|
| 50 m² | 3.5 kW | 5 kW | 6,000 kWh | £520 | £430 |
| 75 m² | 5.3 kW | 7 kW | 9,000 kWh | £780 | £644 |
| 100 m² | 7.0 kW | 7 kW | 12,000 kWh | £1,040 | £859 |
| 150 m² | 10.5 kW | 12 kW | 18,000 kWh | £1,560 | £1,289 |
| 200 m² | 14.0 kW | 14 kW | 24,000 kWh | £2,080 | £1,718 |
Tips before you buy
- Insulate first. Loft and wall insulation and draught-proofing cut the heat the home needs, and let a smaller heat pump run at a lower flow temperature.
- The efficiency (SCOP) matters most. A well designed system with big radiators or underfloor heating runs at a low temperature and makes more heat for each kWh.
- Ask your supplier for a heat pump tariff, and check whether solar panels and a battery can cover some of the power.
- A heat pump works best when it runs steadily at a low output, rather than in short bursts at a high one. Set the heating to a constant, lower temperature.
- Ask for a quote from MCS-certified installers with a heat loss survey, and check for the Boiler Upgrade Scheme grant, which is available in England and Wales.
Tools & materials for this project
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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 here. A SCOP of 3.6 or better, or a cheap heat pump tariff, makes it equal or cheaper.
What is the SCOP of a heat pump?
The seasonal coefficient of performance: the heat made over the year for each unit of electricity used. A good heat pump with a low flow temperature has a SCOP of 3.5 or more.
How much does it cost to run a heat pump?
For the 100 m² home, about £1,040 a year at 26p per kWh and a SCOP of 3, or £600 on a 15p tariff. The gas boiler for the same home costs about £859 at 6.3p.
Do I need bigger radiators for a heat pump?
Often, yes. A heat pump runs at a lower flow temperature, so a radiator gives out less heat than it does with a boiler. See the radiator size calculator for the output to look for.
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Estimates only. Results assume standard materials and simple shapes. Supplier products vary, and the final call on quantity, structure and safety is yours. Check your local code and your supplier's guidance before ordering.