There's a simple sum behind your heating bill: your boiler's gas input in kilowatts, multiplied by the hours it runs, multiplied by your gas unit rate. Here's how to work it out for your own home — and why the answer is usually lower than the headline figure.
Gas central heating is billed by the kilowatt-hour (kWh) of gas you burn, not by the heat you get out. So the headline formula is straightforward:
Cost per hour ≈ boiler gas input (kW) × hours run × gas unit rate (pence per kWh)
The key number people get wrong is the boiler's input. The figure printed large on a combi — say "30kW" or "35kW" — is usually its central heating or hot water output. The gas input (the energy it actually draws from the meter) is a little higher, because no boiler is 100% efficient. A modern A-rated condensing boiler runs at roughly 90% efficiency, so a boiler putting out 24kW of heat is pulling in around 26–27kW of gas. You'll find the gross input rating on the data badge inside the casing or in the manual, often written as "Qn" in kW.
Here's the part the simple formula hides: your boiler almost never runs at full input. Modern combi and system boilers modulate — they throttle the flame up and down to match demand. Once your home reaches temperature, the boiler drops to a fraction of its maximum and cycles on and off. So a "30kW" boiler does not burn 30kW of gas for every hour the heating is on. Over a typical winter hour it might average a third to a half of its rated input. Treat the full-input sum as a worst-case ceiling — your actual cost per hour is usually well below it.
Let's price a standard combi boiler. Under the current Ofgem price cap, the gas unit rate in 2026 is in the region of 6–7p per kWh — but caps change every quarter and tariffs vary, so check your own bill or in-home display for the exact figure. We'll use 6.5p here.
Take a boiler with a gross gas input of around 26kW:
| Scenario | Gas used | Cost (at 6.5p/kWh) |
|---|---|---|
| Full input, one hour (worst case) | 26 kWh | ≈ £1.69 |
| Modulating at ~50% over the hour | 13 kWh | ≈ £0.85 |
| Modulating at ~33% over the hour | ~8.6 kWh | ≈ £0.56 |
So the same boiler can cost anywhere from roughly 56p to £1.69 an hour depending on how hard it's working — and on a mild day, when it's just topping up the warmth, you'll sit near the bottom of that range. Add the daily standing charge for gas (a fixed amount whether or not the heating is on) and you have the full picture. To price your own boiler, swap in your gross input rating and your real unit rate.
Quick tip: watch your gas meter or smart in-home display. Note the kWh reading, run the heating for an hour, then read it again. The difference is exactly what that hour cost in kWh — multiply by your unit rate for the cash figure. This real-world test beats any formula.
A common misunderstanding is that the unit rate itself scales with the size of your home — that a five-bed house somehow pays a higher pence-per-kWh than a flat. It doesn't. The unit rate is set by your tariff and the price cap, not by your property. A mansion and a studio on the same tariff pay the identical price per kWh of gas.
What does change with size is the number of kWh you burn. A larger, draughtier, poorly insulated home loses heat faster, so the boiler runs longer and harder to hold temperature — more kWh, same price each. That's why insulation, draught-proofing, thermostat settings and how long the heating is on matter far more to your bill than the boiler's badge rating. A well-insulated large home can easily cost less to heat per hour than a cold, leaky smaller one.
One safety note: anything beyond the basics above — the gas valve, pipework, flue, sealed circuit, pressure relief valve or removing the casing — is strictly for a Gas Safe registered engineer. Never attempt gas work yourself, and always check an engineer's registration at gassaferegister.co.uk. If you ever smell gas, leave the property and call the emergency line on 0800 111 999.
Knowing your running cost is one half of budgeting; the other is what happens when the boiler breaks down. A failed pump, a leaking heat exchanger or a faulty PCB can mean a sizeable repair bill on top of your day-to-day gas. Boiler cover spreads that risk into a fixed monthly cost, with repairs carried out by registered engineers. Whether it's right for you depends on your boiler's age and your appetite for risk — our guide on whether boiler cover is worth it walks through the maths. When you're ready, you can compare boiler cover from a selected panel of providers and weigh up the options side by side.
As an upper estimate, multiply your boiler's gross gas input in kW by your unit rate. For a ~26kW boiler at around 6.5p/kWh that's roughly £1.69 at full tilt — but because boilers modulate, the real figure over an hour is often nearer 55p–90p. Check your own boiler's rating and your tariff's unit rate for an accurate number.
No. The unit rate is fixed by your tariff and the price cap and is the same regardless of property size. A larger or less well-insulated home simply burns more kWh, so the total bill is higher — but each kWh costs the same.
Output is the heat the boiler delivers; input is the gas it draws from the meter. Because boilers are roughly 90% efficient, input is a little higher than output. Use the gross input rating (often shown as "Qn" on the data badge) for cost calculations.
Under the current price cap it's broadly in the region of 6–7p per kWh, but it changes each quarter and varies by tariff and region. Always check your latest bill or in-home display for your exact rate, and add the daily standing charge to get the full cost.
Yes. Lowering the central-heating flow temperature so the boiler condenses efficiently, using the thermostat and timer sensibly, bleeding radiators, and improving insulation all cut the kWh you burn. For a new low-carbon system, the Boiler Upgrade Scheme may offer a grant towards a heat pump.
This article is general information, not financial or technical advice. Figures are indicative 2026 GBP estimates and depend on your tariff, boiler and usage. We compare a selected panel of providers, not the whole market, and may earn a commission. Always use a Gas Safe registered engineer for gas work.
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