Comparing a heat pump with a gas boiler fairly means comparing the cost of the same warmth — the useful heat delivered into the house — not the cost of the fuel each one buys. A boiler wastes some of the gas it burns; a heat pump multiplies the electricity it draws. Do it in delivered heat and the picture is honest. Here it is, at the Ofgem cap for 1 July – 30 September 2026: 26.11p per kWh electricity, 7.33p gas.
Running cost by house size
These figures assume a typical condensing gas boiler (about 85% efficient in the real world) against an air source heat pump on two radiator scenarios: existing average radiators, and radiators upgraded for low-temperature running. All costs are per year, for heating and hot water combined.
| Home (gas use) | Gas boiler | Heat pump, existing rads | Heat pump, upgraded rads |
|---|---|---|---|
| Flat (6,500 kWh) | £476 | £497 | £424 |
| Terraced (9,500 kWh) | £696 | £727 | £620 |
| Semi (12,000 kWh) | £880 | £918 | £783 |
| Detached (17,000 kWh) | £1,246 | £1,301 | £1,110 |
| Large detached (23,000 kWh) | £1,686 | £1,760 | £1,501 |
One more figure belongs in every row: dropping gas removes the gas standing charge, worth about £106 a year. Subtract that from the heat pump columns and the "existing radiators" case lands roughly level with the boiler, while the "upgraded radiators" case pulls clearly ahead — from around £150 a year in a flat to nearly £300 in a large detached.
The pattern hiding in the table
Notice that on existingradiators the heat pump's advantage shrinks as the house gets bigger. That is not a quirk. At a modest SCOP the electricity cost climbs almost as fast as the gas it replaces, while the fixed standing-charge saving stays the same £106 and so counts for less. On upgraded radiators the opposite happens — a high SCOP means each extra unit of heat is genuinely cheaper than gas, so the bigger the heating demand, the more you save. Radiator sizing matters most, precisely, in the homes that use the most heat.
Where the boiler still wins
On small, unchanged radiators the heat pump has to push its water hot, its SCOP drops toward 2.5, and at that efficiency 26.11p electricity buys heat at about 10.4p a unit — dearer than gas at 7.33p. In that case the boiler is cheaper to run, full stop. No amount of marketing changes the arithmetic; only bigger emitters or a cheaper tariff does. This is the case most installer calculators quietly leave out, and the one our running cost calculator shows you plainly.
What moves the number
- Flow temperature — set by radiator size. The biggest lever by far.
- Your tariff — a heat-pump or EV tariff with cheap overnight units widens the gap.
- The price cap — the electricity-to-gas price ratio changes every quarter and can tip marginal homes either way.
- Insulation — a warmer fabric needs a lower flow temperature, lifting SCOP.
- Hot water — a well-insulated cylinder heated overnight keeps the average efficiency up.
The headline, then, is not "heat pumps are cheaper" or "boilers are cheaper". It is: with the right radiators and a sensible tariff, a heat pump beats gas in almost every UK home; with small radiators on a standard tariff, it may not. The gap in a typical semi swings from saving £200 to costing £80, on the radiators alone. Run your own house through the numbers before you decide.