What Is CoP?
The Coefficient of Performance (CoP) is a ratio that expresses a heat pump’s efficiency. A CoP of 3.0 means the system produces 3 kW of thermal energy for every 1 kW of electrical energy consumed. In other words, it’s 300% efficient — or three times as efficient as using electricity directly in an immersion heater.
CoP vs SCOP
CoP is an instantaneous measurement — it tells you the efficiency at a specific moment in time under specific conditions. SCOP (Seasonal Coefficient of Performance) is the more useful figure for homeowners, as it measures average efficiency over a full heating season accounting for variable outdoor temperatures. SCOP is used on energy labels and for Boiler Upgrade Scheme applications.
What Is a Good SCOP?
- SCOP above 4.0: Excellent (A+++ energy rating)
- SCOP 3.5–4.0: Very good (A++ to A+++)
- SCOP 3.0–3.5: Good (A+ to A++)
- SCOP below 3.0: Below modern standards — consider upgrading
How Temperature Affects CoP
CoP decreases as outdoor temperature drops — there is less heat energy in cold air to extract. At 7°C outside, a typical ASHP might achieve CoP 4.0. At -7°C, the same unit might achieve CoP 2.5. This is why heat pumps work best in the UK’s mild maritime climate compared to, say, Canada or Scandinavia.
Flow Temperature and CoP
Higher flow temperatures (hot water flowing to radiators) reduce CoP significantly. Every 5°C increase in flow temperature reduces CoP by approximately 10%. This is why heat pumps work most efficiently with underfloor heating (35–40°C) compared to standard radiators (50–60°C).
Maintaining Your CoP
Annual servicing is essential to maintain the CoP you paid for. A dirty evaporator coil can reduce CoP by 5–10%. Low refrigerant charge reduces it further. An engineer will check and restore these during an annual service visit.
Sources: the Catapult trial and electricity unit rate.
