Bar shows the load as a share of the planned unit’s output. An estimate for planning only: an MCS certified installer must size the system with a room-by-room heat loss calculation.
Formula and breakdown
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This heat pump size calculator estimates the heat output in kW a heat pump needs to keep a home warm on a cold design day, from either your yearly gas or oil use or a heat loss figure from a survey, for homeowners in England, Scotland, Wales and Northern Ireland. It uses the outdoor design temperatures in the MCS heat pump standard and checks a planned unit against the MCS rule that it must cover the full heat load.
Quick answer: A heat pump size calculator multiplies your home’s heat loss coefficient in W/K by the gap between indoor temperature and the outdoor design temperature. Example: 8,200 kWh of space heating over 2,000 degree days gives 171 W/K; at 20 °C indoors and London’s MCS design temperature of minus 1.7 °C that is about 3.7 kW.
What Is a Heat Pump Size Calculator?
A heat pump size calculator is a tool that estimates the peak heat a building loses on a cold design day, which is the minimum output its heat pump must deliver. In plain words: heat loss coefficient in watts per degree times the difference between indoor and outdoor design temperatures gives the design heat load in watts.
The heat loss coefficient (W/K) describes how many watts leak out through walls, roof, floor, windows and draughts for every degree of difference between inside and outside. A surveyor finds it room by room; an existing home’s fuel use gives a useful first estimate, because the heat the boiler delivered over a heating season equals the heat lost over the same period.
How Do You Use the Heat Pump Size Calculator?
- Choose your method: yearly gas or oil use, or a design heat loss from a survey.
- For the fuel method, enter yearly kWh from your statements and your boiler’s efficiency.
- Enter yearly hot water heat and the heating degree days for your area.
- Pick your nearest city to fill in the MCS outdoor design temperature, and the 99% or 99.6% level, or type your own.
- Enter the average indoor temperature you want on the coldest day.
- Add the output of the heat pump you are considering at that outdoor temperature, its SCOP and your electricity price to see the size check and running cost.
How Do You Work Out Heat Pump Size From Gas Usage?
Multiply yearly gas kWh by boiler efficiency, subtract hot water, then divide by degree days times 24 to get the heat loss coefficient. Example: 12,000 kWh x 85% = 10,200 kWh of useful heat; minus 2,000 kWh of hot water leaves 8,200 kWh of space heating; 8,200,000 Wh / (2,000 x 24) = 171 W/K. At a 21.7 K design difference the load is 171 x 21.7 = 3,707 W, about 3.7 kW.
The degree days and hot water figures are example values, because they vary by region, household and year, so use local degree-day data and your own hot water estimate if you have them. The method is a planning check. Old fuel bills include any rooms heated to unusual temperatures, windows left open or a boiler that ran inefficiently, so the result can differ from a survey. Our heat pump running cost calculator takes the same yearly heat and compares it with gas.
What Outdoor Design Temperature Should You Use?
MCS MIS 3005-D gives UK external design temperatures from CIBSE Guide A, such as minus 1.7 °C for London at the 99% level and minus 3.0 °C at 99.6%. The MCS heat pump design standard lets the installer choose Column A or B depending on the site. The table shows each city’s values and the resulting load for an example home with a heat loss coefficient of 150 W/K kept at 20 °C.
| Location | 99% design temperature | Load at 99% (150 W/K) | 99.6% design temperature | Load at 99.6% (150 W/K) |
|---|---|---|---|---|
| Plymouth | minus 0.2 °C | 3,030 W | minus 1.5 °C | 3,225 W |
| Belfast | minus 1.5 °C | 3,225 W | minus 3.2 °C | 3,480 W |
| Cardiff | minus 1.5 °C | 3,225 W | minus 3.1 °C | 3,465 W |
| London | minus 1.7 °C | 3,255 W | minus 3.0 °C | 3,450 W |
| Manchester | minus 2.7 °C | 3,405 W | minus 4.5 °C | 3,675 W |
| Birmingham | minus 3.2 °C | 3,480 W | minus 5.1 °C | 3,765 W |
| Edinburgh | minus 3.2 °C | 3,480 W | minus 5.4 °C | 3,810 W |
| Glasgow | minus 3.5 °C | 3,525 W | minus 5.9 °C | 3,885 W |
The same home needs about 28% more output in Glasgow at the colder level than in Plymouth at the milder one. MCS also sets indoor design temperatures by room: 21 °C for living rooms, 18 °C for bedrooms, halls and kitchens, and 22 °C for bathrooms, so the heat pump size calculator’s whole-house average of 20 °C is an example value to adjust for your rooms.
Why Does Heat Pump Size Matter?
A heat pump must deliver at least 100% of the calculated heat load at the design temperature without help from a backup heater, under clause 3.4.1 of MIS 3005-D. Too small, and rooms will not reach temperature on the coldest days. Much larger than needed, and the unit usually costs more to buy and install. The heat pump size calculator warns when the planned unit is below 100% or above 150% of the estimated load.
Use the output at your design temperature and flow temperature, not the headline rating. Manufacturers’ data sheets list output at several outdoor and flow temperatures, and a unit sold as one size may give less on a frosty day at a high flow temperature. MCS advises avoiding high temperature heat pumps unless the system needs a flow temperature above 55 °C.
How Much Will a Heat Pump of This Size Cost to Run?
Yearly electricity equals space heating plus hot water divided by the SCOP: 10,200 kWh of heat at an example SCOP of 3 needs 3,400 kWh, which costs £894.88 at the Ofgem cap of 26.32p per kWh for October to December 2026 (direct debit). The Ofgem price cap announcement also confirms VAT is removed from electricity bills from 1 October 2026 to 31 March 2027. To add standing charges and compare tariffs, try our electricity cost calculator.
Who Should Size and Install a Heat Pump?
An MCS certified installer should carry out a room-by-room heat loss calculation to BS EN 12831-1, as MIS 3005-D requires, before choosing the unit, radiators and cylinder. The Boiler Upgrade Scheme offers £7,500 towards an air source or ground source heat pump in England and Wales, and GOV.UK directs applicants to MCS certified installers. The electrical supply, isolator and controls must be installed by a qualified electrician and meet BS 7671. Treat this heat pump size calculator’s figure as a way to check a quote, not as a design.
Worked Example: Checking a Quote for a Semi in London
Example: a 1930s semi-detached house in London used 12,000 kWh of gas last year with an 85% efficient combi boiler, of which about 2,000 kWh went on hot water. Local degree days are taken as 2,000 for the example. An installer quotes a 5 kW air source heat pump that gives 5 kW at minus 2 °C outside and a 45 °C flow.
- Space heating: 12,000 x 0.85 minus 2,000 = 8,200 kWh.
- Heat loss coefficient: 8,200,000 / (2,000 x 24) = 170.8 W/K.
- Design difference: 20 minus (minus 1.7) = 21.7 K, using London’s 99% figure.
- Design load: 170.8 x 21.7 = 3,707 W, about 3.71 kW.
- The quoted 5 kW is 135% of the estimate, above the 100% minimum and below the 150% alert, so the size looks reasonable to discuss with the installer’s own heat loss survey.
- Running cost at SCOP 3: (8,200 + 2,000) / 3 = 3,400 kWh, or £894.88 a year at 26.32p.
Other rough tools, such as our energy consumption calculator, help you see how the new electricity demand fits alongside your existing appliances.
Frequently Asked Questions
What size heat pump do I need for a 3-bedroom house?
There is no single answer, because size depends on heat loss, not bedrooms. Two similar 3-bedroom homes can need very different outputs if one has cavity wall insulation and new windows. Use your yearly gas use or a heat loss survey to estimate the kW, then confirm it with an MCS installer.
How do I estimate heat pump size from my gas bill?
Multiply yearly gas kWh by boiler efficiency, subtract hot water, and divide by degree days x 24 to get the heat loss coefficient in W/K. Multiply that by indoor minus outdoor design temperature for the peak load in watts. It is a planning estimate, not a design.
What outdoor design temperature should I use in the UK?
MCS MIS 3005-D lists external design temperatures from CIBSE Guide A for UK locations. London is minus 1.7 °C at the 99% level and minus 3.0 °C at 99.6%; Glasgow is minus 3.5 °C and minus 5.9 °C. Your installer chooses the column for your site.
Is it bad to oversize a heat pump?
A unit far larger than the heat load usually costs more to buy and install. MCS requires the heat pump to cover at least 100% of the calculated heat load, so the aim is the smallest unit that meets it at your design temperature and flow temperature.
Does the heat pump size include hot water?
The heating design is based on the space heating load at the design temperature. Hot water adds yearly energy and the heat pump must reheat the cylinder; MCS requires a heat pump used for hot water to reach at least 55 °C flow at design conditions. Your installer sizes the cylinder separately.
Who can size and install a heat pump in the UK?
Use an MCS certified installer, who must calculate heat loss to BS EN 12831-1 under MIS 3005-D. GOV.UK points Boiler Upgrade Scheme applicants, who can get £7,500 in England and Wales, to MCS certified installers. Electrical work must be done by a qualified electrician to BS 7671.
Checked October 2026 by the Solaxyra Editorial Team. Sources: MCS: MIS 3005-D:2025, GOV.UK: Boiler Upgrade Scheme, Ofgem: price cap October to December 2026.