What size heat pump do I need?

How the sizing works

Heat pump capacity is sized to the property's design heat loss — the rate at which the building loses heat at the coldest outside temperature you typically see. For the UK that's roughly -3°C external against 21°C internal. The kW figure on the unit needs to match or slightly exceed the heat loss, with a small margin for hot water and recovery from setback.

Heat loss by property age

Older properties lose heat faster because fabric efficiency was lower when they were built. Pre-1900 solid wall homes lose around 100 W/m² of floor area at design conditions. Post-2003 builds, with cavity wall insulation, double or triple glazing, and good loft insulation as standard, lose around 35 W/m². The calculator applies the band you select, then modifies by your current EPC.

EPC band as a heat loss modifier

Two homes of the same age and size can have very different heat losses depending on how the fabric has been maintained or upgraded. A pre-1930 detached at EPC C (good loft insulation, modern double glazing, cavity insulation added) loses materially less heat than the same property at EPC E. The calculator adjusts the baseline by your current band — improving your EPC before installing the heat pump reduces the kW you need and improves running costs.

BUS grant eligibility

The Boiler Upgrade Scheme (BUS) gives £7,500 off the installed cost of an air source heat pump (or £7,500 for ground source). It covers England and Wales only. To be eligible you must own the property and be replacing a fossil fuel heating system (gas, oil, LPG, electric); social housing is excluded. The work is done by an MCS-certified installer, who claims the grant on your behalf and takes it off the quote. An EPC is no longer part of it — the requirement for a valid EPC with no outstanding loft or cavity wall insulation recommendations was removed on 28 April 2026. Insulation still affects how well a heat pump performs, and an installer will raise it, but it is not a condition of the grant. Applications close 31 December 2029.

What this calculator doesn't do

The figure here is indicative. A proper installer survey runs a full room-by-room heat loss calculation (sometimes called an MCS heat loss calculation), checks radiator/emitter compatibility, sizes the hot water cylinder, and considers electrical supply capacity. Use this as a starting point before requesting installer quotes.

Frequently asked questions

What size heat pump do I need for a UK home?

Most UK homes need 5–12 kW of air source heat pump capacity. A modern three-bedroom semi at ~90 m² sizes around 5–6 kW. A pre-1930 four-bedroom detached at ~140 m² with EPC D usually needs 10–12 kW.

How much does a heat pump cost in the UK?

Under the Boiler Upgrade Scheme the median installed cost of an air source heat pump was £12,908 in 2026 Q2 (April to June), with the middle half of installs between £11,128 and £15,627 before the grant. Cost rises with size: the median for a 6-8 kW install in 2025/26 was £12,164, and for 10-12 kW £13,943. The £7,500 grant takes the middle half to £3,628-£8,127. Source: Boiler Upgrade Scheme (BUS) monthly statistics, England and Wales: July 2026 (Department for Energy Security and Net Zero, published 27 August 2026).

Am I eligible for the £7,500 Boiler Upgrade Scheme?

The scheme covers England and Wales. You need to own the property, be replacing a fossil fuel heating system, and have the install done by an MCS-certified installer; social housing is excluded. There is no EPC requirement — it was removed on 28 April 2026. Applications close 31 December 2029.