COP, SCOP and SPF
All three describe how much heat a heat pump delivers for each unit of electricity. The Energy Saving Trust defines the Coefficient of Performance (CoP) as the heat produced per unit of electricity used, so a CoP of 3.0 means three units of heat for one of electricity. Every heat pump has a published datasheet CoP, measured under specific test conditions.
Real homes are not a test rig. Outside temperature changes through the year, and the heat pump works harder when it is colder. So the Trust points to the Seasonal Coefficient of Performance (SCoP) or Seasonal Performance Factor (SPF), which averages efficiency across the whole year. It adds that installers must calculate the SPF from the system design for your home, using the average temperatures at your location and the size of your radiators, and should share the calculation before work starts.
| Term | What it measures | Use it for |
|---|---|---|
| COP | Heat out per unit of electricity at one test condition | Comparing machines on a datasheet |
| SCOP / SCoP | Seasonal average across a typical year | Predicting running cost for your design |
| SPF | Seasonal performance factor, averaged over a year | Judging how the system actually performed |
The glossary entries are on the glossary page, and the broader mechanism is in how a heat pump works.
What homes actually achieve
The Energy Systems Catapult Electrification of Heat project installed 742 heat pumps and monitored them for a year. Among 428 air source units, the median SPFH2 was 2.93 (interquartile range 2.67 to 3.19) and the median on a second measure, SPFH4, was 2.78 (range 2.55 to 3.05). The report also cites an earlier trial, the Renewable Heat Premium Payment scheme, at 2.44 on the SPFH4 basis. The Energy Saving Trust, using its own wording, says typical systems come out at around 3.1.
Two cautions from the same report. In 91.9% of properties the in-situ SPFH2 was below the designed SCOP, with a median gap of 17.9%. And there was no significant difference in SPF by house type or age. In other words, the gap between design and reality is mostly about the system and how it is run, not the age of the building.
Efficiency by outside temperature
| Outside temperature | Median 30-minute COP (air source) |
|---|---|
| 10°C | 3.37 |
| -3°C | 2.37 |
| -5°C | 2.3 |
Source: Energy Systems Catapult, Dec 2024. The ground source median at -5°C air temperature was 3.1 in a small sample, because the ground stays warmer than the air, which is part of why ground source systems tend to be more efficient. The air source vs ground source guide sets out the cost trade-off.
Why flow temperature matters
Efficiency falls as the water temperature the heat pump must produce rises. The trial found a median COP of 3.5 when producing 35°C water, 2.38 at 55°C and 2.02 above 65°C. Across the monitored air source units the mean operating flow temperature was 39.7°C. Heat pumps include weather compensation, which lowers the flow temperature when it is milder outside, and the Energy Saving Trust says a correctly set heating curve is one of the main levers on running cost. Bigger radiators let the system run cooler, which is the reason installers propose them. The controls guide and sizing guide go deeper, and the underfloor heating guide covers the low-temperature end.
Turning efficiency into pounds
Electricity use is heat demand divided by the seasonal figure. This is an illustrative calculation, not a forecast: it takes a home needing 12,000 kWh of heat a year and the Ofgem cap electricity rate of 26.32p/kWh for 1 October to 31 December 2026, excluding standing charges.
| Seasonal figure | Electricity used | Annual cost at 26.32p |
|---|---|---|
| 2.5 | 4,800 kWh | £1,263 |
| 3.0 | 4,000 kWh | £1,053 |
| 3.5 | 3,429 kWh | £902 |
| 4.0 | 3,000 kWh | £790 |
Every step in efficiency matters, but so does the price of a unit. Put your own tariff into the calculator.
What your efficiency means for your bill
No details needed. Change any figure to match your own quote or bills. Results are indicative — an MCS installer’s heat-loss survey and written quote are what count.
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Difference per year at these prices.
Prices: Ofgem cap 1 October to 31 December 2026 — electricity 26.32p/kWh, gas 7.97p/kWh, gas standing charge 29.68p/day (GB average, Direct Debit); heating oil 114.8p/litre including 5% VAT (109.3p before VAT; BoilerJuice UK average for 1,000 litres, read 28 September 2026); 10.35 kWh per litre assumed. No VAT on household electricity from 1 October 2026 to 31 March 2027, which flatters heat pumps in this window. LPG prices vary too much to default — use your own bills. Illustrative, not a quote.
How to read an installer’s figure
- Ask whether the number is a datasheet COP, a calculated SCOP for your design, or a measured SPF from another home.
- Ask for the flow temperature it assumes. A high design flow temperature means a lower seasonal figure.
- Treat an unexplained efficiency promise as marketing. A real one comes with the heat-loss calculation behind it.
- Compare quotes on the quotes checklist, not on the efficiency headline alone.