What size heat pump do I need?
Property age + floor area + EPC band → recommended kW capacity, installed cost range, and the £7,500 Boiler Upgrade Scheme grant offset where you qualify.
Your property
Recommended heat pump
Updates as you change the inputs.
- Recommended capacity
- — kW
- Heat loss (design)
- — W
- Installed cost (range)
- £—–£—
- BUS grant
- £—
- Net cost after grant
- £—–£—
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). You're eligible if you own the property, you're replacing a fossil fuel heating system (gas, oil, LPG, electric), and your property has a valid EPC. Your EPC must show no outstanding recommendations for loft or cavity wall insulation — these need to be addressed first if flagged. The scheme runs to 2028 and is administered by MCS-certified installers, who claim the grant on your behalf.
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?
Installed air source heat pumps cost £8,000–£14,000 in 2026 before grants. With the BUS £7,500 grant applied, net cost is typically £500–£6,500.
Am I eligible for the £7,500 Boiler Upgrade Scheme?
You qualify if you own the property, you're replacing a fossil fuel heating system, you have a valid EPC with no outstanding insulation recommendations, and the install is by an MCS-certified installer.