Heating Cost Calculator

A modern condensing gas boiler delivers about 90% of the energy in the gas as heat; the rest leaves through the flue. So 12,000 kWh of heat needs 13,333 kWh of gas. An oil boiler at 85% needs 14,118 kWh, which is about 1,364 litres of heating oil.

A heat pump moves heat from the outside air rather than making it. Its coefficient of performance (COP) is the units of heat it delivers per unit of electricity. At a COP of 3.2, 12,000 kWh of heat takes only 3,750 kWh of electricity.

At these prices, electricity costs 3.3 times as much per kWh as gas. Allowing for boiler losses, a heat pump beats a gas boiler on running cost once its seasonal COP is above about 3.0. Below that, it costs more to run.

• COP 2.5: £1,263.36 a year — £200 more than gas
• COP 3.2: £987.00 a year — £75.67 less than gas
• COP 3.5: £902.40 a year — £160.27 less than gas

A heat pump is most efficient when it heats water to a lower temperature. Well-insulated homes with larger radiators or underfloor heating let it run cooler, which raises the COP. Old radiators sized for a hot boiler flow force it to work harder.

Ask your installer for the design flow temperature and expected seasonal COP, and enter that figure rather than the best-case number from a brochure.

The table prices the heat pump at the standard unit rate. Some suppliers offer cheaper rates for heat pump owners or cheaper overnight periods. At 15p per kWh, for example, the same 3,750 kWh would cost £562.50. Change the rate above to match any tariff you are offered.

An Energy Performance Certificate lists estimated space heating and hot water demand in kWh. Or take annual gas use from your bill and multiply by 0.9 to allow for boiler losses. Insulation lowers demand for every system, so it saves money whatever you heat with.

Heat demand should include hot water as well as space heating. A combi boiler heats water on demand; a heat pump heats a cylinder, usually to a lower temperature than a boiler would, with a periodic higher-temperature cycle to control bacteria. If you are replacing a combi, find space for a cylinder in your plans.

Oil is bought in bulk deliveries rather than billed per unit, and its price moves more sharply than the gas and electricity price caps. Buying in summer, joining a local buying group and keeping the tank topped up before winter all help. Enter the price from your last delivery rather than a national average.

Gas and oil boilers should be serviced every year, and heat pumps benefit from regular checks too. Servicing is not in these running costs, so add your service plan price when comparing systems over 15 years.

• A room-by-room heat loss calculation, not a guess from floor area
• The design flow temperature and expected seasonal COP
• Which radiators need upgrading, and the cylinder size
• MCS certification, which grant schemes require

If you are replacing a broken boiler, the running cost here tells you what each option costs every year after installation. Put the installed price of each option next to the 15-year figures in the table, and the cheaper choice over the life of the system usually becomes clear.

A boiler swap usually costs far less to install than a heat pump. In England and Wales the Boiler Upgrade Scheme grant has covered £7,500 of an air source heat pump; check current eligibility before getting quotes. Our guide to heat pump vs boiler goes through the efficiency argument in more detail.

If you move off gas completely, you also stop paying the gas standing charge — a saving this comparison does not count. A heat pump raises your electricity usage, which you can check with the electricity cost calculator, and pairs well with the solar panel calculator for spring and autumn.