By the Home Savings Calculator team · 29 August 2026 · 7 min read
A boiler burns fuel to make heat, so it can never produce more heat than the energy it consumes — a good condensing boiler manages around 90%, an older one nearer 70%. A heat pump does not make heat, it moves it, gathering warmth from outside air and concentrating it indoors.
That means a heat pump routinely delivers three or more units of heat per unit of electricity. Comparing the price of a unit of gas with the price of a unit of electricity ignores this entirely, which is why the comparison has to be done on delivered heat.
Put your annual heat demand into the heating cost calculator. It converts demand into fuel or electricity for each system, so gas, oil and heat pump are priced on the same heat rather than the same kilowatt-hour.
The coefficient of performance is heat out divided by electricity in. A COP of 3.5 means 3.5 kWh of warmth per kWh of power. Multiply the electricity rate by the gas efficiency and divide by the COP and you have the real comparison.
COP is not fixed. It falls as the outside temperature drops and as you ask for hotter water, which is why the seasonal figure across a whole year matters more than the headline number on the datasheet. Ask for the seasonal figure, not the peak.
Where gas is cheap relative to electricity, a mediocre COP loses. If your installation ends up running at 2.5 because the radiators are undersized and the flow temperature has to be high, the arithmetic can tip back the other way.
Poorly insulated homes push this the wrong way twice: they need more heat, and they need it hotter, which is exactly the condition where a heat pump is least efficient. That is the real reason insulation is the standard advice before a heat pump, not environmental piety.
Heat pumps work best delivering gentle heat over long periods through large radiators or underfloor pipes. A house set up for a boiler — small radiators, high flow temperatures, intermittent bursts of heat — is the wrong shape for one.
Fixing that is part of the cost. A quote that does not mention radiator sizing has either surveyed the house and found it already suitable, or has not surveyed it.
A heat pump usually costs several times a like-for-like boiler replacement, even after grants. Running-cost savings rarely repay that gap quickly on their own, so the decision is generally about replacing a boiler at end of life, not scrapping a working one.
Where it does get interesting is alongside solar, because a heat pump turns surplus generation into heat instead of exporting it at the low rate.
