Heat Pump Sizing Calculator

How this calculator works

This calculator works in ranges rather than single figures, because real quotes, yields and tariffs genuinely vary that much. It starts from typical UK price, output and savings ranges drawn from published 2026 industry guides and official scheme documentation, then scales them by the quantities you enter and adjusts them for each choice you make below. The result is a low-to-high band that a realistic outcome is likely to fall within, not a promise of the figure you will achieve.

UK homes typically need around 50 to 70 watts of heat pump capacity per square metre at an average insulation standard, so a 100 square metre three bedroom home lands at roughly 5 to 7 kW, matching the official picture in which the average UK installation is an 8 kW unit once hot water and a margin are considered. Annual space heating demand for the same home runs roughly 9,500 to 14,000 kWh, with hot water adding around 1,500 to 2,500 kWh on top depending on household size.

Heated floor area (square metres)

Add up the floor area of the rooms you actually heat, across all storeys. A typical UK three bedroom semi is around 90 to 110 square metres, a two bedroom flat 60 to 75, and a four bedroom detached 130 to 160. Floor area is the biggest single driver of heat demand, though insulation determines how much heat each square metre actually loses.

Insulation standard

Insulation moves the answer more than anything else: a well insulated three bedroom home may need only a 5 kW heat pump while the same home poorly insulated needs 7.5 kW or more. Because a smaller heat pump is cheaper to buy and runs more efficiently at lower flow temperatures, insulation work done before the heat pump pays for itself twice, and installers will often decline to fit a system in a home losing heat faster than a sensibly sized unit can replace it.

Location climate

Heat pumps are sized for the coldest design day in your area, typically around minus 2 to minus 4 degrees Celsius in southern England but several degrees colder in exposed northern and Scottish locations, which pushes the required capacity up by around 10 percent. Altitude and exposure matter too, which is one reason a proper survey beats any calculator.

What else affects the figures

Real sizing is a room-by-room heat loss calculation to the MCS standard, accounting for wall construction, glazing, air tightness, room-by-room temperatures and the design outdoor temperature, and an MCS installer must produce one before quoting. Oversizing is not the safe option it appears: an oversized heat pump cycles on and off, wearing components and cutting efficiency, which is why installers size close to the calculated load rather than adding boiler-style headroom. Hot water is usually served by the same unit via the cylinder rather than by extra capacity, and a buffer or volumiser may appear on the design for good hydraulic reasons rather than as padding.

Certification, planning and grid connection

If a calculation here prompts a project, the practical next step is an MCS-certified installer’s survey: MCS installers must carry out a room-by-room heat loss calculation to the industry standard before quoting, which turns the broad ranges on this page into a design for your actual home, and MCS certification is a condition of the Boiler Upgrade Scheme grant. Most domestic heat pump installations fall under permitted development subject to siting and noise rules, with the usual exceptions for listed buildings and conservation areas, and 0 percent VAT applies to residential installations until March 2027.