TradeCalculator.co.uk
Heat Pump Sizing Calculator — What Size Heat Pump Do I Need?
Calculate the correct heat pump capacity for your UK home. Covers air source (ASHP) and ground source (GSHP) heat pumps with MCS guidance.
Total heated floor area (all floors)
Enter your electricity price in pence per kWh (Ofgem cap 1 Jul–30 Sep 2026 = 26.11p/kWh) for a running cost estimate
Simplified DHW sizing margin (default 3 kW). MCS sizes the pump to space-heating loss; hot water is usually met by cylinder reheat with diversity, not stacked on peak load — set 0 to size on space heating alone.
Full-load-equivalent space-heating hours (default ~1,800, region-dependent). Used only for the indicative running-cost estimate.
Daily hot water heating hours (default 3) for the indicative running-cost estimate.
Next up: related pages
How We Calculate This
Our heat pump sizing calculator gives a quick estimate of your property's space-heating heat loss and an indicative heat pump capacity in kW. It is a simplified W/m² rule-of-thumb, not an MCS design — a full MCS (Microgeneration Certification Scheme) room-by-room heat loss calculation is required for BUS grant eligibility (see the note below).
The calculation method
Space-heating loss (kW) = (Floor area × Heat loss rate × Property factor) ÷ 1000
The headline recommendation then adds an adjustable hot water allowance (default 3 kW) on top of the space-heating loss. This is a deliberate simplification: a proper MCS design (to MIS 3005-D, with heat loss to BS EN 12831) sizes the heat pump to the space-heating design load and meets hot water by cylinder reheat using diversity, rather than stacking a separate kW on top of the peak heating load. Set the hot water allowance to 0 in Advanced Options to size on space heating alone.
Heat loss rates (W/m²)
Indicative rule-of-thumb bands by insulation level (industry sizing guides; always defer to a measured MCS survey):
- Poor insulation (pre-1920 / solid walls): 120-180 W/m²
- Average insulation (1920-1980): 80-120 W/m²
- Good insulation (post-1990, cavity & loft): 50-80 W/m²
- Excellent (recent new build / deep retrofit): 25-45 W/m²
Running cost estimate
The running cost is indicative only. It is estimated from a full-load-equivalent space-heating run time (default ~1,800 hours per year, region-dependent) and the heat pump's seasonal Coefficient of Performance (COP). ASHP typically achieves a seasonal COP of about 3.0, GSHP about 4.0. The electricity cost is calculated from your entered price per kWh — the Ofgem price cap for 1 July to 30 September 2026 is 26.11p/kWh for electricity.
Important note
This calculator provides a rough estimate. A full MCS-compliant heat loss survey (room-by-room calculation to MIS 3005-D, with heat loss to BS EN 12831) is required before installation and for BUS grant eligibility. The survey accounts for specific building fabric, window areas, ventilation rates, and local design temperatures.
Frequently Asked Questions
A typical 3-bedroom semi-detached house (90–110 m²) with average insulation has a space-heating design heat loss of roughly 8–12 kW, which is the figure MCS sizes the heat pump to. This calculator then adds a small adjustable allowance (default 3 kW) for hot water, so its headline recommendation will read a little higher (about 11–15 kW for this example) — set the hot water allowance to 0 to size on space heating alone. Well-insulated properties may need as little as 6–8 kW of space-heating capacity; poorly insulated homes may need 12–16 kW or more. A full MCS-compliant heat loss survey is essential before installation. Compare this with gas boiler sizing using our Boiler Size Calculator.
Air source heat pumps (ASHP) are cheaper to install (typically £8,000-15,000 including BUS grant) and suitable for most UK homes. Ground source heat pumps (GSHP) are more efficient (higher COP) and cheaper to run, but installation costs are higher (£15,000-25,000+) and they need significant garden space. ASHP are the most popular choice in the UK.
Often, yes. Heat pumps operate at lower flow temperatures (35–50 °C vs 60–80 °C for gas boilers), which means radiators need to be larger to deliver the same heat output. A heat pump installer will calculate the radiator sizes needed — use our Radiator Sizing Calculator to check individual room requirements. In well-insulated homes, existing radiators may be adequate. Underfloor heating is ideal for heat pumps — see our Underfloor Heating Pipe Calculator for pipe quantities.
The UK Boiler Upgrade Scheme provides grants of £7,500 towards the cost of an air source or ground source heat pump installation (and £5,000 for a biomass boiler in eligible rural off-gas-grid homes). From 21 July 2026, eligible off-gas-grid properties on oil or LPG can claim £9,000 for an ASHP or GSHP. The scheme was extended to 2030 by the Boiler Upgrade Scheme (England and Wales) (Amendment) Regulations 2026. The installation must be done by an MCS-certified installer, and the grant is applied directly by the installer, reducing your upfront cost.
In UK conditions, a well-installed air source heat pump typically achieves a seasonal COP (SCOP) of 2.8-3.5, meaning for every 1 kWh of electricity used, it produces 2.8-3.5 kWh of heat. Ground source heat pumps achieve 3.5-4.5 SCOP. Actual performance depends on flow temperature, installation quality, and climate.
Related Calculators
Was this calculator helpful?
Last updated: June 2026
Verified against UK standards · estimates only, confirm with your supplier.