Reading a Heat Pump Spec Sheet — COP, SCOP and HSPF for Cambridge Buyers

On a typical spec sheet you’ll see…

Figure	What it measures	Use it for
COP at A7/W35	Efficiency at 7°C outdoor air, 35°C water, at one moment	Comparing models like for like
COP at A2/W35 or A-7/W35	The same, at colder outdoor temperatures	Seeing how a model copes with cold
SCOP (35°C)	Seasonal average for underfloor-style water temperatures	Low-temperature systems
SCOP (55°C)	Seasonal average for radiator-style water temperatures	Higher-temperature systems
HSPF	A US seasonal measure in BTU per watt-hour	Rarely relevant in the UK

The “A” number is the outdoor air temperature and the “W” number is the water temperature. So A2/W35 means 2°C outside, 35°C water.

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Which figure predicts your bills?

SCOP at your design flow temperature. If your system is designed for 45°C, the right number sits between the brochure’s 35°C and 55°C figures. Your installer should estimate it for your home, not just quote the best brochure number.

Then:

Annual electricity (kWh) = annual heat demand (kWh) ÷ SCOP

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Adjusting for Cambridge

Spec-sheet SCOPs use a standard European “average” climate. Cambridge is drier than average but frostier than most of England, with around 52 air-frost days a year at the NIAB weather station. In practice:

• Expect a real-world SCOP slightly below the brochure figure at the same flow temperature, especially if the unit spends many nights defrosting.
• Pay attention to the cold-weather COP (A2 or A-7). A model that holds its efficiency well at low temperatures suits Cambridge.
• Low flow temperatures matter more here than in milder cities, because they protect efficiency on cold days.

A well-designed Cambridge system usually achieves a seasonal efficiency of around 3.0–3.5 with radiators, higher with underfloor heating.

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A worked example

A Chesterton semi needs about 12,000 kWh of heat a year. The installer designs for 45°C and estimates a SCOP of 3.3.

• Electricity: 12,000 ÷ 3.3 ≈ 3,640 kWh
• At about 26p per kWh: ≈ £950 a year
• At a heat pump tariff averaging about 18p: ≈ £655 a year
• Gas boiler equivalent, including standing charge: ≈ £1,170

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HSPF in one paragraph

HSPF is an American rating mostly used for air-to-air systems. It’s expressed in BTU per watt-hour, so the numbers look bigger: an HSPF of 10 is roughly a seasonal ratio of 2.9. If you’re comparing an air-to-air unit for a small Cambridge flat, ask for the European SCOP instead.

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Sanity checks

• A quoted SCOP above 4 for an older Cambridge home with radiators deserves questions.
• A SCOP below 2.8 suggests a high flow temperature; ask whether larger radiators would help.
• The installer’s estimate should state the flow temperature it assumes.

Related: flow temperature and design temperature · running costs through a Cambridge winter