For homeowners

What size heat pump do I need? Estimate it from your gas use in five minutes

Published October 2026 · 5 min read

Before anyone comes round with a laser measure, you can get a decent first stab at your home's heat loss from something you already have: your gas use. It takes one cold day, three numbers and a bit of long division. No app needed, though we do have one.

Two warnings up front. You need the gas used on one particular cold day, which a smart meter or your supplier's app will show; a quarterly bill on its own won't. And this gives you one rough number for the whole house. It will tell you whether you're a 4 kW house or a 12 kW house. It won't tell you which radiators need changing. More on that at the end.

The numbers you need

1. Gas used on one properly cold day, in kWh. A smart meter in-home display, your supplier's app or their website shows daily use. Pick a day in January or February when the heating was on all day. If you can only see pounds, turn the day's cost into pence, take off the standing charge, and divide by your unit rate in pence per kWh. £6.40 of gas at 8p a kWh is 640 ÷ 8 = 80 kWh.
2. The average outside temperature that day. Any weather site's history page will give it. Average, not the overnight low.
3. What your thermostat was set to. Probably 19 to 21°C.

One more number helps, and you already have it: the gas you use on a summer day, when the heating is off. That is close enough to your hot water and cooking, and it comes off the top. Average a few summer days if you can; one is fine to start.

The sum, with a real-ish house

Say the house burned 90 kWh of gas on a day that averaged 3°C outside, with the thermostat at 20°C, and its summer days show about 6 kWh.

1. Take off hot water and cooking. Showers and the hob aren't heating the house. 90 − 6 = 84 kWh of gas went into heating.
2. Allow for the boiler not being perfect. A condensing boiler turns roughly 85% of the gas into heat in the radiators; the rest goes up the flue. An older boiler manages less, which is why the calculator below lets you change it. 84 × 0.85 = 71 kWh of actual heat.
3. Spread it over the day. 71 kWh over 24 hours is about 3.0 kW. That's the average rate the boiler was putting heat into the house on a 3°C day.
4. Stretch it to a really cold day. Heat pumps are sized for the coldest weather you'd reasonably expect, not an average winter day. Installers in much of England design for around −3°C outside; colder further north and up hills (our design temperatures guide explains). The maths is a ratio: how far inside is from outside on the design day, divided by how far it was on your day.
   3.0 × (20 − (−3)) ÷ (20 − 3) = 3.0 × 23 ÷ 17 ≈ 4 kW. (With a calculator you get 4.06; the method is nowhere near precise enough for the decimal to mean anything.)

So, on these assumptions, the house loses about 4 kW on the worst day. Not 12. And not the 30 kW the old combi boasts on its badge, which is a hot-water number and doesn't tell you how much heat your house needs.

Or let the page do the long division

Same sum, your numbers. It runs in your browser and sends nothing anywhere.

Where this falls down

This is a rough estimate, and several things can throw it out. Some make the figure too low, others too high. It is useful for getting a sense of the numbers, not a substitute for a proper heat loss survey.

• It assumes the house was warm all day. If the heating only ran morning and evening, the house spent hours cooler than the thermostat and used less gas than a heat pump would need to hold it at 20°C round the clock. That makes the figure too low.
• Gas isn't the only thing heating your house. Sunshine through the windows, the people in it, the oven, the telly and the lights all add warmth without burning gas, as does a log burner or an electric heater if you have one. A sunny day makes the number look lower than it really is, and a design calculation generally doesn't count on any of it.
• It's one number for the whole house. Heat pumps run cooler than boilers, so the question that actually decides the job is whether each room's radiator can shift enough heat at a lower flow temperature. A whole-house figure can't answer that.
• Boiler efficiency cuts both ways. A non-condensing boiler from the nineties might manage 70%, not 85%. Leave 85 in for a boiler like that and the figure comes out too high; put the right number in and it drops. If you are not sure, the boiler's age is the best clue.
• Design temperature isn't one number. Coastal Cornwall and the Pennines are not the same cold day. Your installer will use the figure for your area and altitude.

4 kW of heat loss is not a 4 kW heat pump

One more thing before you take the number to an installer. The capacity printed on a heat pump is measured on a mild day, 7°C outside with cool water going round. At −3°C, pushing hotter water and stopping now and then to defrost, the same machine delivers less. So a house that loses 4 kW is usually fitted with a unit badged 5 or 6 kW, and that is correct sizing, not an installer over-egging it. Your installer should pick the model from the manufacturer's output at your design temperature and flow temperature, not from the badge.

What to do with the number

Keep it in your back pocket. When an installer quotes you, put their room-by-room heat loss next to your estimate. The two won't necessarily match, but a substantial difference is worth a conversation. If they haven't measured a single room, that's your first question: an MCS-certified installer is required to calculate the heat loss of every room before sizing a heat pump.

Our plain-English guide to heat loss and the heat pump readiness guide fill in the rest.

Quick questions

Why not just match the boiler's size?

Because a combi's 30 kW is sized to make hot water fast, not to heat the house. Houses need far less: the sum above is the kind of number to expect, and a big draughty detached house might be two or three times it. Size a heat pump off the boiler badge and you'll pay for a machine that spends its life switching itself on and off.

My bill only shows pounds, or cubic metres.

Pounds: turn the day's gas cost into pence, take off the standing charge, and divide by your unit rate in pence per kWh. £6.40 at 8p is 640 ÷ 8 = 80 kWh. Cubic metres: the conversion is printed on the bill, roughly 11.2 kWh per cubic metre for most of the UK. If you have an older imperial meter that counts in hundreds of cubic feet, multiply its reading by 2.83 to get cubic metres first.

I don't have a smart meter.

Read the meter at the same time two days running during a cold snap and take the difference; that is one day's use. Do it over several cold days and average them if you can. A quarterly bill won't do the job: three months of gas against one average temperature hides exactly the cold-day detail the sum depends on.