COP, SCOP and HSPF — Heat Pump Efficiency Numbers Explained for Bristol
Last reviewed: 2 October 2026
Heat pump brochures are full of efficiency figures. This guide explains which one actually predicts your running costs, why it changes with flow temperature and climate, and what's realistic for a Bristol home.
Three acronyms, one question
Every heat pump efficiency figure answers the same question: how much heat do you get for each unit of electricity? A figure of 3 means three units of heat per unit of electricity. The differences are about when and how it’s measured.
COP: a snapshot
Coefficient of Performance is efficiency at one moment, under one set of conditions — for example, 7°C outside with 35°C water. Brochures quote COPs at test points, and the numbers often look impressive (4.5 or 5) because the test conditions are mild.
Useful for comparing models under identical conditions. Not useful for predicting a year of bills.
SCOP: the one that matters
Seasonal Coefficient of Performance is the average over a whole heating season, across mild days, cold days and defrosts. It’s the figure that sets your running costs:
Yearly electricity = yearly heat needed ÷ SCOP
A Bristol terrace needing 12,500 kWh of heat with a SCOP of 3.5 uses about 3,570 kWh of electricity — around £940 a year at 26p per kWh. At SCOP 2.8, it’s about 4,460 kWh, or £1,175.
Manufacturers’ SCOPs are tested to a European standard for an “average” climate at fixed water temperatures, usually 35°C and 55°C. Your installer should estimate a SCOP for your home at your design flow temperature.
HSPF: mostly an American number
Heating Seasonal Performance Factor is the US equivalent of SCOP, but measured in British thermal units per watt-hour rather than as a simple ratio, and mainly used for air-to-air systems. You’ll see it on some imported or air-to-air product sheets. Roughly, dividing HSPF by 3.41 gives a comparable seasonal ratio. For UK radiator systems, ignore it and ask for SCOP.
What changes your real-world SCOP
- Flow temperature — the biggest factor. Designing for 45°C instead of 55°C can lift SCOP from about 2.8 to 3.5. See flow temperature.
- Climate — milder air means less work. This is where Bristol does well.
- Sizing — oversized units short-cycle in mild weather and lose efficiency.
- How you run it — steady operation beats on-off use.
- Hot water — heated to a higher temperature than space heating, so it runs at a lower efficiency. Bristol’s hard water can scale the cylinder coil and make this worse over time.
What’s realistic in Bristol
Bristol is among the mildest big cities in England. Filton averages a January overnight low of about 2.4°C and only around 33 air-frost days a year. That means a heat pump spends most of the winter in conditions where it’s efficient. A well-designed system here typically achieves:
| Home and design | Realistic SCOP |
|---|---|
| Newer house, 40°C design | 3.8–4.2 |
| Interwar semi, 45°C design | 3.4–3.8 |
| Victorian terrace, 45–50°C design | 3.1–3.6 |
| Large Georgian townhouse, 50–55°C design | 2.8–3.2 |
Anything above about 3.0 beats a 90% efficient gas boiler on running cost at October 2026 prices.
Questions to ask your installer
- What SCOP do you expect for my home, at what design flow temperature?
- Is that based on my heat loss calculation or the brochure?
- What could we change to improve it?
Related: running costs in Bristol · how a heat pump works
See what a heat pump would look like in your Bristol home
The mechanism is the same in every UK home; the design is specific to yours. Our team carries out a free in-home survey including a full room-by-room heat-loss calculation, identifies the system size and configuration that fits your property, and provides a written quote with the £7,500 BUS grant already deducted.
You'll see the actual figure you'd pay — not an estimate — and the projected efficiency for your specific install.