Show the formula and working
This heat pump size calculator estimates the kW output an air source or ground source heat pump needs for your home, using either floor area and insulation level or your real yearly gas use. It is built for UK and European homeowners who want a sensible figure before they speak to installers.
Quick answer: the heat pump size calculator multiplies floor area by 35 to 100 W/m² depending on insulation, or divides yearly gas use x boiler efficiency by about 2,200 full load hours. A 95 m² older home using 11,500 kWh of gas a year needs about 4.4 kW, so a 5 or 6 kW unit.
What Is a Heat Pump Size Calculator?
A heat pump size calculator estimates the peak heat output, in kilowatts, that a heat pump must deliver to keep your home warm on a cold winter day.
It uses two methods. The floor area method multiplies heated floor area by a heat loss rate in watts per square metre, from 35 W/m² for a new build to 100 W/m² for a poorly insulated home, then divides by 1,000. The gas use method takes your yearly gas kWh, multiplies by boiler efficiency to find the heat actually delivered, and divides by the number of equivalent full load hours in a year.
When you enter gas use, the heat pump size calculator shows that figure as the main result, because it reflects how your home really behaves rather than an average for its age.
How Do You Use the Heat Pump Size Calculator?
- Enter the heated floor area in the heat pump size calculator in square metres or square feet. Add up every heated floor, not just the footprint.
- Choose the insulation level that best matches your home, from new build to solid walls with little insulation.
- Find your yearly gas use in kWh on an annual statement or your supplier’s app and enter it.
- Enter your current boiler’s efficiency. 85% is a fair figure for a working condensing boiler; older non condensing boilers are often lower.
- Leave full load hours at 2,200 unless you have a reason to change it. 2,000 to 2,500 is a common UK range.
- Read the estimated size, the two methods side by side, and the next common unit sizes to look at.
What Size Heat Pump Do I Need?
Most UK homes need a heat pump between 5 and 12 kW: a well insulated three bedroom semi often needs 5 to 8 kW, while a larger or poorly insulated detached house can need 10 to 16 kW.
Bedrooms are a poor guide. Two houses of the same size can differ by double in heat loss because of wall type, glazing and draughts. That is why our heat pump size calculator asks for insulation level and gas use, not bedroom count.
Hot water adds less than people expect, so the heat pump size calculator leaves it out. A cylinder is reheated between heating cycles, so most installers size mainly on space heating, with the cylinder sized separately for your household.
Why Is the Gas Use Method Usually More Accurate?
The gas use method is usually more accurate because your gas bill records how much heat your home really used over a whole year, including draughts, habits and the local climate that an average W/m² figure cannot know.
In the heat pump size calculator, dividing heat delivered by full load hours spreads the year’s heat over the hours a heating system would run if it ran flat out. A UK home uses roughly 2,000 to 2,500 such hours a year. If you also cook with gas, subtract a few hundred kWh. If your gas also heats water from a combi, the estimate will be slightly high, which leaves a safe margin.
Check your current gas spend with the gas bill calculator, which uses the Ofgem price cap of 7.97p per kWh for October to December 2026.
Heat Pump Size by Insulation Level for a 95 m² Home
The table shows the floor area results from the heat pump size calculator for a typical 95 m² UK house at each insulation level.
| Insulation level | Heat loss rate | Estimated size | Common unit to look at |
|---|---|---|---|
| New build or very well insulated | 35 W/m² | 3.3 kW | 5 or 6 kW |
| Good: cavity and loft insulated | 50 W/m² | 4.8 kW | 5 or 6 kW |
| Average older home | 70 W/m² | 6.7 kW | 7 or 8 kW |
| Poor: solid walls, little insulation | 100 W/m² | 9.5 kW | 10 or 11 kW |
Worked Example: Gareth’s Semi in Cardiff
Gareth has a 1960s semi in Cardiff with 95 m² of heated floor, unfilled cavity walls and a 10 year old condensing combi. His last annual statement shows 11,500 kWh of gas. He picks Average older home (70 W/m²) and keeps 85% efficiency and 2,200 hours.
- Floor area method: 95 m² x 70 W/m² = 6.7 kW.
- Heat delivered by the boiler: 11,500 kWh x 0.85 = 9,775 kWh.
- Gas use method: 9,775 kWh / 2,200 h = 4.4 kW.
- Common sizes to look at: 5 or 6 kW.
The two methods differ because Gareth keeps his home at 19°C and heats only the rooms he uses, so the house loses less than the average for its age. The heat pump size calculator shows both so he can question a quote for a 9 kW unit. If he fills the cavity walls first, the needed size falls further.
Does a Heat Pump Need an MCS Heat Loss Survey?
Yes: to claim the Boiler Upgrade Scheme grant in England and Wales, the heat pump must be designed and fitted by an MCS certified installer, who calculates heat loss room by room to the MCS standard MIS 3005 using BS EN 12831 methods.
The grant is £7,500 for an air source or ground source heat pump, with up to £9,000 for eligible homes that used oil or LPG until 31 March 2027, according to Ofgem’s Boiler Upgrade Scheme page. The survey uses a design outdoor temperature for your postcode from CIBSE data, for example -3°C for London and -5.6°C for Glasgow, set out in the MCS heat load calculator documentation.
Use the heat pump size calculator to check that a quote is in the right range, not to order a unit. Any new electrical circuit for the heat pump must be installed by a qualified electrician.
What Happens If a Heat Pump Is the Wrong Size?
An oversized heat pump costs more to buy and can cycle on and off in mild weather, lowering efficiency, while an undersized one relies on its backup electric heater on the coldest days, which turns each kWh of electricity into only 1 kWh of heat.
Radiators matter as much as the figure from any heat pump size calculator. Heat pumps run best at 35 to 50°C flow, so check each room with the radiator output calculator. To estimate yearly running costs at the current price cap, use the heat pump running cost calculator.
How Does Heat Pump Sizing Differ in the UK and Europe?
The method is the same everywhere, but colder design temperatures in much of continental Europe raise the peak load, so a home with the same insulation may need a larger unit in Munich or Warsaw than in Bristol.
Full load hours also change with climate. Colder regions with long heating seasons can see more hours, which lowers the size the gas use method suggests for the same yearly gas use. If you live outside the UK, adjust the hours input in the heat pump size calculator to match local guidance from your installer, and use your own national survey standard. Many EU countries also apply EN 12831 for heat load, with local grant rules that differ from the UK scheme.
Heat Pump Size Calculator FAQs
What size heat pump do I need for a 3 bedroom house?
Most 3 bedroom UK houses need about 5 to 8 kW. A well insulated semi is often nearer 5 kW, while an older home with unfilled cavity walls can need 8 kW. Your gas use gives a better estimate than bedroom count.
How many kW heat pump for 100 m²?
At typical rates of 35 to 100 W/m², a 100 m² home needs about 3.5 to 10 kW. A home with filled cavity walls and a well insulated loft at 50 W/m² needs about 5 kW before an installer’s survey.
Is a bigger heat pump better?
No. An oversized heat pump costs more, may short cycle in mild weather and can run less efficiently. The aim is a unit that just meets the heat loss at the local design temperature, as calculated in an MCS survey.
What are full load hours?
Full load hours are the number of hours a heating system would need to run flat out to deliver a year’s heat. UK homes typically see about 2,000 to 2,500 hours, and the heat pump size calculator lets you change the figure.
Can I use my gas bill to size a heat pump?
Yes. Multiply yearly gas kWh by boiler efficiency, then divide by about 2,200 full load hours. It is a quick sense check that often gives a smaller, more realistic figure than floor area rules.
Do I need to insulate before getting a heat pump?
Insulating first lowers heat loss, so you can fit a smaller and cheaper heat pump with smaller radiators. Loft and cavity wall insulation usually give the quickest return. Your installer may also advise on draught proofing.
Checked October 2026. Sources: Ofgem (Boiler Upgrade Scheme), MCS Heat Load Calculator design conditions, GOV.UK Boiler Upgrade Scheme, Ofgem price cap October to December 2026.