If your heating bills feel unpredictable, one of the best things you can do is learn how to estimate your monthly heating costs yourself.
You do not need to be an engineer to get a useful estimate. In most cases, you can build a solid monthly heating cost estimate from four things:
- your boiler size in kW
- your boiler efficiency
- how many hours it runs
- your gas price per kWh
This matters because heating and hot water account for more than half of annual household energy spending, so even small efficiency differences can have a noticeable impact on your bill. Energy suppliers also bill gas by kWh, which makes it easier to estimate costs once you know how much useful heat your boiler is producing.
If you want the short version, the process is:
Estimated monthly heating cost = gas used in kWh × your gas unit rate
The only challenge is accurately estimating gas usage. That is where boiler size and efficiency come in.
Why boiler size and efficiency matter
A boiler’s size is its power output, usually shown in kW. This tells you how much heat it can deliver when running.
A boiler’s efficiency tells you how much of the fuel you pay for is turned into useful heat. For example, if a boiler is 90% efficient, around 90% of the gas energy is converted to useful heat, with the remaining 10% lost mainly through the flue. Modern condensing boilers are more efficient than older boilers because they recover more heat from exhaust gases that would otherwise be wasted.
That means two homes that need the same amount of heat can still have different gas bills if one boiler is less efficient.
Step 1: Find your boiler size
Boiler size is usually listed in kW on the boiler manual, specification sheet, or model documentation.
Typical domestic boiler outputs often fall into ranges like:
- 24kW to 30kW for many smaller to medium homes
- 30kW to 35kW+ for larger hot water demand or bigger homes
Do not directly confuse boiler size with house size. A larger kW boiler does not automatically mean higher bills. It simply means the boiler can deliver more heat when needed. The cost depends more on how much heat you actually use and how efficiently your boiler delivers it.
Step 2: Estimate your boiler efficiency
If you do not know your exact efficiency rating, you can estimate it based on the type and age of the boiler.
As a rule of thumb:
- modern condensing boilers are typically 90%+ efficient
- older non-condensing boilers can be much lower, sometimes around 70% to 85%, especially as they age
This difference matters. If your boiler is 70% efficient, you are losing far more of the gas you pay for than with a 90% efficient model.
Step 3: Estimate how many hours your boiler runs
This is the part that varies most from home to home.
Your boiler does not usually fire at full output constantly all day. In practice, usage depends on:
- outside temperature
- insulation levels
- thermostat setting
- heating schedule
- size of the heated area
- hot water demand
For a simple estimate, many households start by assuming a daily runtime such as:
- 2 to 4 hours per day in milder weather
- 5 to 8 hours per day in colder months
This is not a universal rule, but it gives you a practical working estimate to model likely monthly costs.
Step 4: Use your gas price per kWh
Your gas bill should show your unit rate in pence per kWh. Ofgem states that gas unit prices are billed in kWh, and for the UK energy price cap from 1 April to 30 June 2026, the typical gas unit rate is 5.93p per kWh, though your actual tariff may differ.
Always use your own tariff where possible. The estimate becomes much more useful when based on your actual unit price.
The basic heating cost formula
A simple way to estimate the monthly heating cost is:
Gas input (kWh) = Heat output needed ÷ Boiler efficiency
Then:
Monthly cost = Gas input × Price per kWh
If you are estimating from boiler size directly, a rough shortcut is:
Monthly cost = Boiler size (kW) × Runtime hours × Days × Price per kWh ÷ Efficiency
Where efficiency is written as a decimal.
Example:
- Boiler size = 24kW
- Efficiency = 90% = 0.90
- Runtime = 3 hours per day
- Days = 30
- Gas price = 5.93p per kWh
First estimate gas input:
24 × 3 × 30 = 2,160 kWh of heat demand at full output
Adjust for efficiency:
2,160 ÷ 0.90 = 2,400 kWh of gas input
Now estimate cost:
2,400 × 5.93p = 14,232p
That is about £142.32 per month at that usage level and tariff.
This is a rough estimate, but it is useful for budgeting and comparisons.
Example: how efficiency changes the monthly cost
Let’s say two homes need the same amount of useful heat: 1,800 kWh per month.
Boiler A: 90% efficient
Gas needed:
1,800 ÷ 0.90 = 2,000 kWh
Cost at 5.93p per kWh:
2,000 × 5.93p = £118.60
Boiler B: 75% efficient
Gas needed:
1,800 ÷ 0.75 = 2,400 kWh
Cost at 5.93p per kWh:
2,400 × 5.93p = £142.32
Difference:
£23.72 per month
This shows why boiler efficiency matters. The less efficient boiler needs more gas to deliver the same heat.
Example: comparing boiler sizes
Now imagine two boilers running for the same amount of time.
Boiler 1
- 18kW
- 90% efficiency
- 3 hours per day
- 30 days
- 5.93p per kWh
Gas input:
18 × 3 × 30 ÷ 0.90 = 1,800 kWh
Monthly cost:
1,800 × 5.93p = £106.74
Boiler 2
- 30kW
- 90% efficiency
- 3 hours per day
- 30 days
- 5.93p per kWh
Gas input:
30 × 3 × 30 ÷ 0.90 = 3,000 kWh
Monthly cost:
3,000 × 5.93p = £177.90
In reality, a larger boiler will not always run at full output for the same pattern as a smaller one, but this example shows how output and runtime combine to affect the estimate.
A quicker way to estimate household gas use
If you do not know your runtime, another practical method is to estimate from the monthly gas use in kWh.
Ofgem says a typical household in England, Scotland, and Wales uses about 11,500 kWh of gas per year, which averages roughly 958 kWh per month, although winter heating months are usually much higher and summer months much lower.
If your winter month usage is, for example, 1,500 kWh, and your gas rate is 5.93p per kWh, then:
1,500 × 5.93p = £88.95
That gives you a direct monthly gas usage estimate without needing to infer boiler runtime.
This is where your Gas Usage to Cost Calculator is especially useful, because it already converts gas usage in kWh and price per kWh into daily, monthly, and yearly cost estimates.
What can make your estimate less accurate?
No estimate is perfect because real heating demand changes constantly.
Key variables include:
- weather and outside temperature
- how well insulated the property is
- whether the boiler cycles on and off
- hot water use alongside space heating
- thermostat settings
- radiator balancing and controls
- whether the boiler is well-maintained
This is why it is usually best to treat the result as a planning estimate rather than an exact bill forecast.
How to reduce monthly heating costs
If your estimate looks high, there are several practical ways to reduce it.
Improve insulation
Energy Saving Trust notes that insulation and draught-proofing reduce heat loss, helping homes stay warmer for longer and cutting the amount of gas needed.
Lower the thermostat slightly
Even a small reduction can make a noticeable difference over time, especially across a whole heating season.
Use timers and room controls
Heating only the rooms you need, when you need them, can reduce wasted boiler runtime. Your own gas calculator page already recommends timers and thermostatic radiator valves as practical cost-saving steps.
Keep the boiler serviced
A well-maintained boiler is more likely to operate efficiently and safely over time.
Upgrade an old boiler
Modern condensing boilers are more efficient than older non-condensing systems, so replacing a very old boiler can reduce the gas needed to produce the same heat.
FAQ about monthly heating costs
How do I estimate my monthly heating costs?
Multiply your estimated gas usage in kWh by your gas unit rate. If you are estimating from the boiler, use the boiler size, runtime, and efficiency to first calculate the likely gas consumption.
Does boiler size affect heating costs?
Yes. A larger boiler can use more gas if it runs at higher output for longer, though the actual cost depends on heat demand and controls, not just boiler size.
How does boiler efficiency affect gas bills?
A more efficient boiler converts more of the gas you pay for into useful heat, so less energy is wasted.
What is a good boiler efficiency rating?
Modern condensing boilers are typically 90% or more efficient, while older boilers can be much lower.
What is the current gas price per kWh in the UK?
Under Ofgem’s typical price cap values for 1 April to 30 June 2026, gas is 5.93p per kWh, though your own tariff may differ.