Heat Pump Sizing Calculator — What Size Heat Pump Do I Need?

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.