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Quick answer
A boiler’s kW rating is its maximum output, not what it uses all the time. A 24kW combi running flat out would burn roughly 26 to 27kWh of gas an hour once efficiency is allowed for, which at the October 2026 capped rate of 7.97p per kWh is a little over £2 an hour. In practice a modern boiler modulates down to a fraction of its maximum once the house is warm, so real hourly use is usually far lower. The best way to know your own figure is to read your gas meter before and after a set period of heating.
Key takeaways
- The kW figure on a boiler is maximum heat output; actual gas use depends on how hard it is working.
- Gas is billed in kWh, so input kWh roughly equals output kWh divided by efficiency.
- Modern condensing boilers modulate, running at low output for long periods once rooms reach temperature.
- Hot water on a combi draws full or near-full output while the tap runs, which is why long showers cost more than people expect.
- A simple meter-reading test gives you your household’s real consumption per hour of heating.
What the kW rating actually means
A boiler described as 24kW, 30kW or 35kW can deliver that much heat at most. On a combi, the highest output is usually reserved for hot water, because heating tap water instantly takes far more power than keeping radiators warm. The heating side is often limited to a lower output, set during installation to match the house. That is why an oversized combi does not automatically mean higher heating bills, but a badly set up one can cycle on and off and waste gas.
Turning kW into gas used and pounds spent
Energy suppliers bill gas in kWh. If a boiler delivers 24kW of heat at around 90% efficiency, it needs about 26.7kW of gas input. Run for one hour at full output, that is roughly 26.7kWh. At 7.97p per kWh, the capped unit rate from October 2026, that works out at about £2.13 an hour.
That figure is a ceiling rather than a typical hour. Once the house is up to temperature, a boiler might run at 20% to 40% of its maximum, or cycle on for a few minutes at a time. The cost of an hour of steady-state heating can therefore be well under a pound in a reasonably insulated home.
Why hot water can surprise you
Combi boilers heat water on demand. When you run a shower, the boiler typically ramps up to its hot water output straight away and stays there until the tap closes. A ten-minute shower on a 30kW combi can use something like 5kWh of gas. It is not a reason to worry, but it explains why households with teenagers or long daily baths see higher gas use than the heating alone would suggest.
How to measure your own boiler’s gas use
- Choose a time when no other gas appliance (hob, fire) will be used.
- Read your gas meter and note the figure. On most meters you only need the black or white digits before the decimal point.
- Run the heating for exactly one hour and read the meter again.
- Convert the units to kWh. Metric meters show cubic metres; your bill shows the calorific value and conversion factor used. Roughly, one cubic metre is around 11kWh.
- Multiply the kWh by your unit rate.
Repeat on a cold day and a mild day and you will see how much the weather changes your consumption.
What makes the biggest difference to gas use
Flow temperature. Condensing boilers are most efficient when the return water is cool. Many are left at high settings that stop them condensing. Our post on boiler flow temperature settings explains the trade-off.
Controls. A room thermostat and TRVs stop the boiler running just because the timer says so. See thermostat and TRV settings for winter.
System condition. Sludge, air and scale all make the boiler work harder. A magnetic system filter and correct inhibitor levels help keep the system clean.
Servicing. A service confirms combustion is set correctly and the heat exchanger is clean, which keeps efficiency close to what the manufacturer intended. Book an annual boiler service if yours is due.
Book with Gasmaster
Worried your boiler is using more gas than it should? Gasmaster Plumbing & Heating, Gas Safe registered (220932), services and repairs boilers across Stoke-on-Trent, Newcastle-under-Lyme and Biddulph. Book through our boiler service booking page.
Frequently asked questions
Does a bigger boiler use more gas?
Not necessarily. A larger boiler only uses more gas if it is running at higher output. A correctly set up boiler matched to the house can be efficient even with a higher maximum rating, but a badly oversized one may short-cycle.
Is it cheaper to leave the heating on low all day?
It depends on the house. We look at the evidence in leave the heating on low all day.
How much gas does a boiler use on standby?
Modern boilers with electronic ignition use no gas on standby. Older boilers with a permanent pilot light burn a small amount continuously.
Why is my gas bill higher when the weather is milder?
Higher bills in mild weather usually point to hot water use, controls running the heating unnecessarily, or a boiler that is not condensing. A service and a look at the controls is a good first step.
Sources
- Ofgem, October to December 2026 price cap announcement (ofgem.gov.uk)
- Energy Saving Trust, heating and hot water guidance (energysavingtrust.org.uk)
- GOV.UK, guidance on gas bills and calorific value
Related services
Abdul runs Gasmaster Plumbing & Heating and has worked on boilers, heating and gas appliances across Stoke-on-Trent since 2005. He is Gas Safe registered (reg. 220932), and these articles come from his day to day work in homes and businesses across Staffordshire, Cheshire and Derbyshire.
