How the comparison works
Both systems have to deliver the same amount of useful heat, so the calculator starts from your annual heat demand and works backwards to the energy each system must buy.
Heat pump cost = heat demand ÷ SCOP × electricity price
A boiler that is 90 percent efficient burns 1.11 kWh of gas for every kWh of heat. A heat pump with a seasonal coefficient of performance (SCOP) of 3.2 draws 0.31 kWh of electricity for the same kWh of heat. The heat pump wins on running cost when the extra price of electricity per kWh is smaller than the extra heat it delivers per kWh.
Finding your annual heat demand
Take the gas you used over a full year in kWh and multiply it by your boiler efficiency. A home that used 13,300 kWh of gas with a 90 percent boiler needed about 12,000 kWh of heat. If your bills show cubic metres instead of kWh, convert them first with the gas m³ to kWh calculator. Hot water counts as heat, so a full year of bills already includes it.
Worked example
The table uses the default inputs: 12,000 kWh of heat, a 90 percent boiler, gas at 0.08 per kWh and electricity at 0.28 per kWh. The only things that change are the heat pump efficiency and the electricity price.
| Scenario | Heat pump SCOP | Heat pump electricity price | Heat pump cost per year | Gas boiler cost per year | Result |
|---|---|---|---|---|---|
| Defaults | 3.2 | 0.28 | 1,050 | 1,067 | 17 cheaper |
| Better efficiency | 3.8 | 0.28 | 884 | 1,067 | 182 cheaper |
| Weak installation | 2.6 | 0.28 | 1,292 | 1,067 | 226 dearer |
| Heat pump tariff | 3.2 | 0.20 | 750 | 1,067 | 317 cheaper |
At the default prices the break-even SCOP is 3.15. Below that number the heat pump costs more to run than the gas boiler, and above it the heat pump costs less. The break-even formula is short enough to do on paper.
0.28 × 0.90 ÷ 0.08 = 3.15
On a heat pump tariff at 0.20 per kWh the break-even drops to 2.25, which is why tariff choice can matter as much as the machine itself.
Why the price ratio decides the result
Divide the electricity price by the gas price and you get the ratio that matters. At 0.28 and 0.08 the ratio is 3.5. A heat pump with a SCOP of 3.2 and a boiler at 90 percent efficiency break even at a ratio of about 3.56, so the default case sits just inside the line. One UK installer comparison using the October 2026 price cap put electricity at roughly 3.3 times the price of gas, as Reonic reports from Ofgem figures. The ratio differs between countries and between tariffs, so use your own bill.
Payback with a grant
Installation cost changes the picture more than running cost does. Take the heat pump tariff case with a 12,000 installation, a 7,500 grant and a 2,500 boiler replacement you would have bought anyway. The net upfront cost is 2,000, the saving is 317 a year, and the simple payback is 6.3 years. Without the grant and without the avoided boiler, the same saving takes more than 37 years to repay 12,000. England and Wales run the Boiler Upgrade Scheme, and other countries have their own programmes, so check the current amount on GOV.UK or your national energy agency.
What the calculator leaves out
Servicing costs, radiator upgrades, a hot water cylinder and electricity from your own solar panels are not modelled. A heat pump also tends to run at lower efficiency when it heats water than when it heats rooms, so a single SCOP is a simplification. If you disconnect from gas, enter the yearly standing charge you stop paying, because that can add a meaningful amount to the saving.
Frequently asked questions
Which SCOP should I enter?
Ask the installer for the design SCOP of your specific house and flow temperature, then enter a figure a little below it. Installed systems commonly land between 2.5 and 4, and homes with oversized radiators or low flow temperatures sit at the upper end.
Can I compare oil or LPG instead of gas?
Yes. Enter the oil or LPG price per kWh in the gas price field and the efficiency of your boiler. Convert litres to kWh using the figure on your supplier's bill.
Does the result include hot water?
It does if your heat demand includes it. A full year of gas bills covers heating and hot water together.
Should prices include VAT?
Yes, use the prices you actually pay, including VAT, so that both sides of the comparison are on the same footing.