Heat Pump Efficiency: SCOP, COP and UK 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 defined by the European standard EN 14825, which sets out the temperatures, part-load conditions and calculation method behind a SCOP figure. The most commonly cited test point 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 a SCOP of 3.5–4.5; for context, products must meet a government minimum SCOP of 2.8 to qualify for grant support, and the average for recently MCS-certified air source units is around 3.2. A figure 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. As a rule of thumb, 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.

If upgrading radiators is not practical right now, a hybrid heat pump offers a middle path: it retains your existing boiler as backup for the coldest days, so the heat pump does not have to run at the high flow temperatures that reduce efficiency. This can be a pragmatic first step for older homes before more fabric improvements are in place.

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 in England and Wales. The scheme is administered by Ofgem and delivered through MCS-certified installers, who apply for the grant on your behalf and discount it from the price. The eligibility rules were relaxed in 2026: a valid EPC is no longer a mandatory requirement (from 28 April 2026), and the earlier condition that the property have no outstanding loft or cavity wall insulation recommendations has been removed. You do still need to own the property and be replacing a fossil-fuel heating system, so check the current criteria before applying — and insulating first will materially improve the heat pump's real-world efficiency.

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.

Once you know which efficiency band to target, our guide to the best air source heat pumps in the UK ranks the top models by SCOP, noise, and installed cost after the BUS grant. To compare air source heat pumps on SCOP figures and system compatibility, browse heat pumps on SmartSolarHomes.

One factor that is increasingly worth noting when comparing models is the refrigerant type. R290 (propane, GWP 3) is replacing R32 (GWP 675) as the leading refrigerant in residential heat pumps, driven by EU F-Gas regulations tightening from 2027. Our R290 heat pump guide explains the regulatory background, which models are available, and how to decide whether refrigerant type should affect your purchase decision.

UK ASHP SCOP comparison: real-world test data

MCS-certified heat pumps are tested to EN 14511 standards, with SCOP measured at defined test points. The two most relevant for UK conditions are A7/W35 (7°C outdoor air, 35°C flow temperature — typical UK mild weather with underfloor heating) and A-7/W35 (minus 7°C outdoor — cold winter conditions).

Note: installed costs are indicative ranges including MCS-certified installation; actual costs vary by property and location.

How flow temperature affects SCOP

The single biggest factor under a homeowner's control is flow temperature — the temperature at which the heat pump sends water to your radiators or underfloor heating. Every 1°C increase in flow temperature reduces SCOP by approximately 2–3%.

• 35°C flow (underfloor heating or oversized radiators): Highest efficiency — SCOP values in the 3.5–5.0 range
• 45°C flow (standard radiators, well-insulated home): SCOP drops to approximately 3.0–4.0
• 55°C flow (older radiators, draughty homes): SCOP drops further to approximately 2.5–3.2
• 65°C+ flow: A heat pump operating at this temperature is rarely more efficient than a modern gas boiler

This is why the BUS grant application process includes an assessment by an MCS-certified installer — the system design must demonstrate that the heat pump can meet heating demand at a flow temperature that delivers meaningful carbon savings.

SCOP and BUS grant eligibility

The Boiler Upgrade Scheme (BUS) requires that installations meet the SAP 10.2 minimum primary energy efficiency threshold. Under SAP 10.2, a heat pump's seasonal performance is modelled at the specific dwelling's design conditions, not just a generic test point.

In practice, this means installations where the home has poor insulation or requires high flow temperatures may not achieve the required performance threshold. The MCS Standard MIS 3005 sets out the minimum design and installation requirements that must be met for a BUS-eligible system, including heat loss calculation, flow temperature design, and cylinder sizing.

The key BUS grant amounts as of 2026: £7,500 for air source heat pumps, £7,500 for ground source heat pumps, and £7,500 for heat pumps on heat networks (subject to any scheme updates — check gov.uk for current figures).