ASHP Sizing Calculator

The heat pump must be sized to meet the whole-house heat loss at the design outside temperature (roughly −3°C to −5.5°C across the UK, from CIBSE Guide A 99.6th-percentile data). This covers fabric losses through walls, roof, floor and windows, plus ventilation losses, plus a domestic hot-water reheat allowance. The MCS heat-pump design standard (MIS 3005-D) requires the heat pump to provide at least 100% of the calculated design heat loss; significant oversizing much beyond ~125% is discouraged as good practice because it harms part-load efficiency.

When outside temperatures are near or below 0°C, frost forms on the outdoor coil. The heat pump periodically reverses to defrost it, reducing effective heating output. A figure of roughly 10% is used here as an indicative rule of thumb — the real loss depends on humidity and the manufacturer — so a margin is allowed when sizing for cold weather.

The balance point is the outdoor temperature below which the heat pump alone cannot meet the full heating demand. Below this temperature, supplementary heating (an electric immersion element, or the gas boiler in a hybrid system) is needed. MIS 3005-D defines the design condition as the coldest ~1% of hours in the year, and requires the heat pump to provide at least 100% of the calculated heat loss at that point. The coldest 1% of 8,760 hours works out at only ~90 hours a year, so sizing to exactly the design load (rather than oversizing the heat pump for those few hours) leaves just that handful of hours where some top-up may be needed.

COP (Coefficient of Performance) is the instantaneous heating efficiency. As the outdoor air cools, the heat pump works harder to extract heat, so COP falls. At +7°C outside with a 35°C flow, COP might be 4.0+; at −5°C with a 45°C flow it can drop to about 2.2–2.5. The seasonal figure (SCOP) is the average across the whole heating season and is higher — typically 3.0–3.5 at a 45°C flow for a UK home (MCS 026 / SAP average climate).

They answer different questions. The COP at design temperature is the worst-case efficiency on the coldest day, used to check the heat pump can still deliver enough heat then. The Seasonal COP (SCOP) is the year-round average and is what running costs should be based on — most heating hours occur at much milder temperatures than the design point, so using the worst-case COP for annual energy would overstate running costs. This calculator uses the SCOP (~3.0–3.5) for the annual electricity, cost and saving figures.

No. Oversizing causes short-cycling (frequent on/off), which reduces efficiency and increases wear. MIS 3005-D requires at least 100% of the design heat loss, and good practice is to avoid oversizing much beyond ~125%. Variable-speed (inverter) heat pumps modulate their output to match demand at part loads, so a correctly sized unit is more efficient than an oversized one.

It depends mainly on floor area and insulation standard, not bedroom count on its own. Running this calculator's own default floor areas through its average-insulation setting (75 W/m², cavity fill and double glazing) with the standard 2 kW hot-water allowance gives these nominal capacities: a flat (60 m²) around 10 kW, a mid-terrace (75 m²) around 12 kW, an end-terrace or semi-detached home (95 m², roughly a 3-bed) around 14 kW, a detached house (130 m²) around 16 kW, and a large 4+ bed detached (200 m²) around 24 kW. Nominal means the badge rating at the +7°C test point, so each figure already carries the headroom needed to still cover the load once the design-temperature and defrost derates are applied. Insulation moves them a lot: the same 95 m² semi needs about 18 kW if poorly insulated (120 W/m²) but only 8–10 kW if well insulated (30–50 W/m²). These are indicative sizing figures from this calculator's rule-of-thumb model, not a substitute for the room-by-room heat-loss survey to BS EN 12831 that a full MCS design requires. Check a proposed capacity against MIS 3005-D with our MCS design compliance calculator, or if you are sizing a ground source system instead, use the ground source heat pump calculator.

Using this calculator’s own defaults for the "Large Detached (4+ bed)" property type with average insulation and the Midlands region: floor area defaults to 200 m² and the average-insulation factor is 75 W/m² (both from the calculator’s own tables), so whole-house heat loss = 200 × 75 × 1.0 (ceiling-height factor) ÷ 1000 = 15 kW. Adding the standard 2 kW hot-water allowance gives 17 kW total peak demand. That 17 kW has to be delivered at the −5°C Midlands design temperature, not at the +7°C test point a heat pump is badged at, so the derate is applied before the capacity is picked: a 15% temperature derate plus the 10% defrost derating that applies below 0°C leaves 75% of nominal, and 17 ÷ 0.75 = 22.7 kW, rounded up to the nearest 2 kW = 24 kW nominal. Working back down, the defrost derating is 24 × 0.10 = 2.4 kW and the indicative capacity at the −5°C design temperature is 24 × 0.85 − 2.4 = 18 kW, which clears the 17 kW peak demand by about 6% and so stays inside the roughly 125% ceiling good practice puts on oversizing at the design condition. A 24 kW nominal figure sits at or above the top of most single domestic units, so at this heat loss it is worth pricing fabric improvements, or two units in cascade, before one large machine. Always confirm the real output from the manufacturer’s capacity table at your own design temperature and flow temperature, since a unit badged 16 kW will not necessarily give 16 kW at −5°C. The design-temperature COP works out at 2.3 and the seasonal COP (SCOP) at 3.14 (the result card rounds this to 3.1), giving an annual electricity use of about 7,866 kWh from a 24,700 kWh total annual heat demand (22,500 kWh space heating at 1,500 equivalent full-load hours, plus 2,200 kWh hot water).

Continuing the 4-bed detached example above: the 24,700 kWh annual heat demand divided by the 3.14 seasonal COP gives about 7,866 kWh of electricity a year. At the Ofgem price cap for 1 July to 30 September 2026 (26.11p/kWh electricity), that comes to about £2,050 a year (£2,053.81 as the calculator reports it) to run the heat pump. The same 24,700 kWh delivered by an 85%-efficient gas boiler at the Ofgem Q3 2026 gas rate (7.33p/kWh) costs about £2,130 a year, so the heat pump saves around £76 a year for this specific house, insulation level and region. A better-insulated version of the same house, or one in a milder region, reaches a higher SCOP and a bigger saving; a poorly insulated house in a cold region can end up costing more to run on electricity than on gas at these rates. Always re-run the calculator’s Advanced Options with your own tariff, since a heat-pump or off-peak electricity tariff shifts this comparison further in the heat pump’s favour.

This page is the most detailed of the heat-pump tools here, though still an estimate rather than an MCS design: whole-house heat loss at the regional design temperature, defrost derating, a design-temp capacity check against peak demand, and the backup/hybrid threshold. For a quicker homeowner-facing estimate that also checks whether your existing radiators and hot-water cylinder will cope, use the Air Source Heat Pump Calculator. To check a proposed heat pump against the MCS MIS 3005-D design rules (oversizing, MCS 020 a) noise limit, room-by-room method), use the MCS design compliance calculator. For full installed cost, the £7,500 Boiler Upgrade Scheme grant and payback, see the heat pump cost calculator. Sizing for Scotland: select Scotland in the Region field above to apply the −5.5°C design temperature (this tool’s approximation of CIBSE Guide A winter data for Edinburgh/Gogarbank, ≈ −5.4°C), which lowers both the design-point COP and the SCOP and so raises the running-cost figures, and sets the backup threshold at −3.5°C. It does not change the recommended capacity or the defrost derating in this model, because both are derived from floor area and insulation level alone. Highland and high-altitude sites are colder than that band, since MCS sizing subtracts 0.6°C per 100 m of altitude, so use a postcode-specific design temperature there.