Use this SPF calculator to find the seasonal performance factor a heat pump needs to match the running cost of a gas boiler.
The calculator starts at the break-even point, where the estimated heat pump and gas boiler running costs are the same. Change the energy prices, boiler efficiency, heat pump SPF or annual heat needed to see how each figure affects the result.
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Use SPF Break Even Calculator
Break-even SPF
3.54
Heat pump per year
£847
Gas boiler per year
£847
Annual difference
About the same
At an SPF of 3.54, the estimated annual running costs are broadly the same.
Spark gap (25p ÷ 6p)
4.17×
Heat pump useful heat (25p ÷ SPF 3.54)
7.06p/kWh
Gas boiler useful heat (6p ÷ 85% efficiency)
7.06p/kWh
Please enter valid numbers greater than zero. Boiler efficiency cannot exceed 100%.
What is the spark gap?
The spark gap is the ratio between electricity and gas prices.
For example, if electricity costs 25p per kWh and gas costs 6p per kWh, electricity is 4.17 times the price of gas.
However, this does not mean that heating a home with a heat pump costs 4.17 times as much.
A heat pump produces several units of heat from each unit of electricity. A gas boiler also loses some of the energy contained in the gas it uses.
The calculator accounts for both before comparing the cost of useful heat.
What is the break-even SPF?
The break-even SPF is the seasonal performance factor a heat pump needs to match the running cost of a gas boiler.
If the heat pump achieves a higher SPF, it will be cheaper to run under the prices entered.
If it achieves a lower SPF, the gas boiler will be cheaper.
Starting at the break-even SPF lets you change the electricity price, gas price, boiler efficiency or heat pump performance and see how each one affects the result.
What does SPF mean?
SPF stands for seasonal performance factor. It measures the performance of an installed heat pump over a longer period, normally a full year.
An SPF of 4 means the heat pump has produced four units of heat for every unit of electricity used.
For running-cost comparisons, SPF is more useful than a manufacturer’s headline COP because it reflects the performance of the installed system over time.
Why boiler efficiency matters
A gas boiler does not turn every unit of gas into useful heat.
This calculator uses 85% as its default boiler efficiency. This means that 85% of the energy bought as gas is assumed to become useful heat.
A well-set-up condensing boiler may perform better. An older boiler, or one operating at high flow and return temperatures, may perform worse.
You can change the figure to test a different assumption, such as 90%.
What does annual heat needed mean?
Annual heat needed is the amount of useful heat delivered into the home over a year. It is not the amount of gas or electricity purchased.
The calculator works out how much gas or electricity each heating system would need to provide that amount of heat.
If you do not know your annual heat requirement, you can leave the default figure in place and use it as an example. Changing this figure will scale the annual costs up or down, but it will not change the break-even SPF.
How the calculation works
- Spark gap = electricity price ÷ gas price
- Heat pump useful heat cost = electricity price ÷ SPF
- Gas boiler useful heat cost = gas price ÷ boiler efficiency
- Break-even SPF = electricity price × boiler efficiency ÷ gas price
- Heat pump annual cost = annual heat needed ÷ SPF × electricity price
- Gas boiler annual cost = annual heat needed ÷ boiler efficiency × gas price
These figures compare energy unit costs only. They do not include standing charges, installation costs, servicing costs or other household energy use.
If you use a time-of-use or variable electricity tariff, enter the average unit price actually paid for the electricity used by the heat pump.
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