Heat pump running costs: a worked example against a gas boiler

Stating the assumptions clearly

To compare running costs meaningfully, we need a home's annual heat demand and the efficiency of each heating system. For this worked example we assume an annual heat demand of 12,000 kWh, a reasonably typical figure for a three-bedroom home, though your own home could be higher or lower.

We compare two SCOP (seasonal coefficient of performance) scenarios for the heat pump, 3.0 and 3.5, representing a less efficient and more efficient real-world installation respectively, against a gas boiler assumed to run at 85% efficiency, a reasonable figure for a modern condensing boiler.

For unit prices, this example uses illustrative figures of 25p per kWh for electricity and 6p per kWh for gas. These are stated assumptions for demonstrating the method only, not current market prices. Always check up-to-date prices with your own supplier or Ofgem's price cap information before drawing conclusions.

All prices and demand figures in this section are illustrative assumptions used purely to demonstrate the calculation method. They are not current energy prices and should not be relied upon for a real cost comparison.

Working through the calculation

For the heat pump, electricity consumed equals heat demand divided by SCOP. At a SCOP of 3.0, that is 12,000 ÷ 3.0 = 4,000 kWh of electricity. At a SCOP of 3.5, that is 12,000 ÷ 3.5 ≈ 3,429 kWh of electricity.

Multiplying by the assumed electricity price of 25p per kWh gives an annual running cost of £1,000 at SCOP 3.0, and approximately £857 at SCOP 3.5.

For the gas boiler, gas consumed equals heat demand divided by boiler efficiency: 12,000 ÷ 0.85 ≈ 14,118 kWh of gas. At the assumed gas price of 6p per kWh, that comes to approximately £847 per year.

What this worked example does and does not show

In this particular illustrative example, the gas boiler and the more efficient heat pump scenario come out fairly close, while the less efficient heat pump scenario costs more. This outcome is extremely sensitive to the exact electricity and gas prices used, which vary over time and by supplier and tariff.

A heat pump achieving a genuinely good real-world SCOP, well-sized radiators, and access to a competitive electricity tariff, including some time-of-use or heat pump specific tariffs available from some suppliers, can shift this comparison meaningfully in the heat pump's favour.

Conversely, an undersized heat pump working harder than expected, or a poorly insulated home with higher heat demand than assumed, will tend to produce a lower real-world SCOP and higher running costs than this example suggests.

Running the numbers for your own home

To get a realistic comparison for your own home, you need your actual annual heat demand, ideally from a proper heat loss survey, a realistic SCOP estimate for your specific property from your installer, and current electricity and gas unit prices from your own tariff or Ofgem's published price cap figures.

Ask your installer for their SCOP estimate specific to your property, based on the heat loss survey, rather than a generic efficiency figure, since real-world performance depends heavily on correct sizing and installation quality.

Results vary considerably by property, installation quality, tariff and weather. This guide shows the method of calculation, not a prediction for any specific home.