Estimate how much a gas boiler costs to run per day, month and year from its heat output, daily hours, gas unit rate, efficiency and standing charge.
Gas boiler · running cost · UK p/kWh
Rated heat output — not the same as gas input when the boiler modulates.
Estimate how long the boiler fires for heating each day.
Running cost = (useful heat ÷ efficiency × gas price) + standing charge, scaled to your usage pattern. An 18 kW boiler running 5 hours a day at 6p/kWh gas and 85% efficiency costs roughly £6.70 per day including standing charge — about £2,430 per year if used every day.
Adjust hours, boiler size and gas price to match your home. This is an estimate, not a meter reading.
Boiler kW rating is maximum heat output, not what it burns every minute. Modern condensing boilers modulate down when less heat is needed, so actual gas use is often lower than this calculator suggests if you enter high hours at full rated output.
Use hours as "equivalent full-output hours" — a rough estimate of how much the boiler actually fires for space heating on a typical day.
Gas standing charge is a daily fee for supply, similar to electricity. It applies whether the boiler runs or not. Over a year, 32p/day adds about £117 — meaningful when comparing tariffs or estimating shoulder-season costs.
When comparing gas suppliers, look at both unit rate and standing charge. A low unit rate with a high standing charge can cost more for moderate users.
Divide daily gas cost by hours of use. An 18 kW boiler running at full output for one hour delivers 18 kWh of heat, but gas input is higher once you account for efficiency — at 85% that is about 21 kWh of gas. At 6p/kWh gas, one hour at full output costs roughly £1.26 in gas alone, before standing charge. Most boilers do not run at full output continuously.
Estimate useful heat per day (kW × hours), divide by boiler efficiency to get gas input in kWh, multiply by your gas unit rate in pence and divide by 100. Add the daily standing charge. Scale to weekly and annual based on days used. The calculator above does this automatically.
Use 85% for a modern condensing boiler in typical operation. Older non-condensing boilers might be 70–80%. Manufacturer "ErP" ratings can be higher than real-world seasonal performance. If you know your annual gas use and heat demand, you can back-calculate a personal efficiency figure.
Yes. The daily standing charge is included in the per-day, monthly and annual totals. You pay standing charge even on days the boiler barely runs, which matters when estimating shoulder-season costs.
Boilers modulate their flame to match heat demand. Rated kW is maximum output, not average. Thermostats, weather compensation and hot-water demand all change how long and how hard the boiler runs. This calculator uses your estimated hours at the rated output as a simplified model.
Per kWh of heat delivered, gas is usually cheaper than direct electric heating because gas unit rates are lower and boilers deliver close to one unit of heat per unit of gas (after efficiency losses). Heat pumps can beat gas on running cost because they move heat rather than generate it, but upfront cost is higher. Compare using our heat pump running cost calculator for a like-for-like annual estimate.
