Heat Pump Efficiency Explained: SCOP, COP, and What They Mean for Running Costs

If you are evaluating air source heat pumps, you will quickly encounter two similar-sounding abbreviations: COP and SCOP. Both express efficiency as a ratio of heat output to electricity input, but they measure very different things — and conflating them is one of the most common ways people end up with misleading estimates of running costs.

COP vs SCOP: the key difference

COP (Coefficient of Performance) is an instantaneous efficiency measurement taken at a specific set of conditions. A heat pump might have a COP of 4.5 at 7°C outside air and 35°C flow temperature — meaning it delivers 4.5kWh of heat for every 1kWh of electricity consumed at those exact conditions. But those conditions change constantly. Outside air temperature drops overnight and in winter; flow temperature requirements change with demand.

SCOP (Seasonal Coefficient of Performance) averages the efficiency across a full heating season, accounting for the range of conditions the heat pump will actually experience. It is a much more useful number for estimating annual running costs. A heat pump with a COP of 4.5 on a mild day might have a SCOP of 2.8–3.2 once you factor in colder days and part-load operation — a significantly different picture.

The A7/W35 comparison point

To make SCOP figures comparable across manufacturers, the industry uses standardised test points. The most commonly cited for UK residential applications is A7/W35 — 7°C outside air temperature, 35°C flow temperature. This reflects a mid-season UK heating scenario reasonably well. When comparing heat pumps, always compare SCOP at A7/W35 rather than at higher outdoor temperatures where every heat pump looks impressive.

At A7/W35, a good air source heat pump achieves SCOP of 3.5–4.5. Below 3.0 at this test point should give you pause. Some manufacturers publish only the SCOP at a milder test point (A10/W35 or A15/W35); if you cannot find the A7/W35 figure, ask for it directly.

How flow temperature affects efficiency and running costs

Flow temperature is the temperature of the water leaving the heat pump and entering your heating system. Every 1°C increase in flow temperature reduces the heat pump's efficiency by roughly 2–3%. This has a significant practical implication: a heat pump running at 55°C flow (to supply old, undersized radiators) might achieve a SCOP of 2.2–2.5, whereas the same unit running at 35°C flow (into underfloor heating or oversized radiators) might achieve 3.5–4.0.

This is why heat pump installations often pair with radiator upgrades or underfloor heating. The heat pump hardware cost is only part of the picture — the emitter system determines whether you achieve the published efficiency figures in practice.

The Boiler Upgrade Scheme and what it requires

The Boiler Upgrade Scheme (BUS) offers £7,500 towards the installed cost of an air source heat pump. The grant is administered through MCS-certified installers and applied at point of sale. To be eligible, your property must have a valid EPC with no outstanding recommendations for loft or cavity wall insulation that have not been addressed.

The grant makes a meaningful difference to payback period — a £12,000 heat pump installation effectively costs £4,500 after BUS funding. Combined with solar panels generating cheap daytime electricity for the heat pump to use, the economics of a heat pump in a well-insulated home in 2026 are significantly better than they were two years ago. See solar panels on SmartSolarHomes for how generation output affects the combined running costs.

To compare air source heat pumps on SCOP figures and system compatibility, browse heat pumps on SmartSolarHomes.