The coefficient of performance is the ratio of heat delivered to electricity consumed. A SCOP of 3.5 means the pump delivers 3.5 kWh of heat for every 1 kWh of electricity. Seasonal COP (SCOP) averages this over a heating season and is the figure to use here.

Why does my boiler efficiency matter?

A gas boiler does not turn all fuel into useful heat. A modern condensing boiler is around 90 percent efficient, an old one nearer 75. The tool divides your heat demand by this efficiency to find the actual fuel burned, which is what you pay for and what emits CO2.

How do I find my annual heat demand?

Check your gas bills for annual kWh and multiply by boiler efficiency to get useful heat, or use a rough guide: a typical UK semi-detached home needs about 12,000 kWh of heat per year, a flat 6,000, a large detached house 20,000 or more.

Why can a heat pump cost more to run even though it is efficient?

Electricity usually costs three to four times more per kWh than gas. A heat pump only saves money when its COP is high enough to overcome that price gap. With a SCOP of 3.5 and electricity at roughly three times the gas price, running costs come out similar; higher COP or cheaper electricity tips it in the pump's favour.

Are the CO2 figures accurate?

They use standard DEFRA emission factors: about 0.183 kg CO2e per kWh for natural gas, 0.247 for heating oil, 0.214 for LPG, and 0.207 for grid electricity. As the electricity grid decarbonises, the heat pump's carbon advantage grows even where running costs are level.

Heat Pump Savings Calculator

Switching from a gas, oil, or LPG boiler to a heat pump changes both your running cost and your carbon footprint, and the two do not always move in the same direction. This calculator works out the real annual cost and CO2e of each system from your heat demand, fuel prices, and the pump’s efficiency.

How it works

Both systems must deliver the same useful heat; the difference is how much energy and money that takes:

boiler fuel used   = heat demand / boiler efficiency
boiler cost        = boiler fuel used × fuel price
boiler CO2 (kg)    = boiler fuel used × fuel emission factor

heat pump elec     = heat demand / SCOP
heat pump cost     = heat pump elec × electricity price
heat pump CO2 (kg) = heat pump elec × grid emission factor

saving = boiler cost − heat pump cost

A heat pump delivering heat at a SCOP of 3.5 uses only a third of the input energy a boiler does, but each unit of electricity costs more than gas, so the result depends on the balance of efficiency and price.

Example and tips

A home needing 12,000 kWh of heat with a 90 percent efficient gas boiler burns about 13,330 kWh of gas. At 7p/kWh that is roughly 933 a year and 2.4 tonnes of CO2. A heat pump at SCOP 3.5 uses about 3,430 kWh of electricity; at 28p/kWh that is about 960 a year but only 0.7 tonnes of CO2. Running costs are similar, yet carbon falls by around 70 percent. Push SCOP up with low-temperature radiators or underfloor heating, or pair with a cheap-rate or solar tariff, to make the pump cheaper to run as well as cleaner.