Compares the annual running cost and operational carbon of an air-source heat pump against a gas boiler meeting the same heat demand: using SCOP, boiler efficiency, unit prices and DESNZ emission factors.
The two axes can disagree: gas is often cheaper to run while the heat pump is far lower carbon. Indicative only. Verify to project tariffs and MCS design.
Indicative comparison only. Always verify against project tariffs and a qualified MCS design.
About this heat pump vs gas calculator
This free heat pump vs gas calculator compares the annual running cost and the operational carbon of an air-source heat pump against a gas boiler that meets the same annual heat demand. Enter your heat demand in kWh per year, the heat pump’s seasonal performance (SCOP), the boiler’s seasonal efficiency, your electricity and gas unit prices, and the carbon factors. It returns the cost and carbon for each system side by side, plus the annual saving on both axes. It is aimed at UK building-services engineers, retrofit assessors and installers weighing up a low-carbon heating swap at feasibility stage.
How the comparison is calculated
Both systems deliver the same heat demand Q. The heat pump’s electricity input is Q ÷ SCOP and the boiler’s gas input is Q ÷ (η/100), where SCOP follows the MCS 026 seasonal basis and boiler efficiency follows the ErP / Boiler Plus seasonal figure (an A-rated condensing boiler is around 90%). Running cost is energy in × unit price, and operational carbon is energy in × the emission factor. The default carbon factors are the UK Government / DESNZ (Defra) Greenhouse gas reporting conversion factors 2025, 0.177 kgCO₂e/kWh for grid electricity (Scope 2 generation) and 0.18296 kgCO₂e/kWh for natural gas (Gross CV basis). The two verdicts are independent: at typical UK tariffs a gas boiler is often cheaper to run while the heat pump is far lower carbon, so the calculator never collapses them into one answer.
Frequently asked questions
Why can gas be cheaper but the heat pump still ‘win’?
Running cost depends on unit prices, which currently favour gas in the UK because electricity costs roughly three to four times as much per kWh. Carbon depends on emission factors, where the decarbonising grid now beats gas by a wide margin. The two axes answer different questions, so the tool reports both.
What SCOP should I use?
SCOP is the heat pump’s seasonal coefficient of performance, heat out divided by electricity in across the year. A well-designed UK air-source system on MCS 026 typically achieves 3.0 to 3.5; the MCS-compliant minimum is around 2.8. Use the manufacturer’s declared SCOP at your flow temperature for the most accurate result.
Does this include grid transmission losses?
No: the default 0.177 kgCO₂e/kWh is the DESNZ generation / Scope 2 factor and excludes transmission & distribution losses (about 8%) and well-to-tank. For a strict consumption-basis comparison, raise the electricity factor accordingly; it is fully editable.