What does COP actually measure?

COP is the ratio of useful heat output (in kW) to electrical input (in kW) at a single fixed operating point. For air-source heat pumps the standard test point is 7°C outside air, 35°C flow temperature — this gives the highest COP because both conditions are mild. A heat pump rated COP 5.0 at A7/W35 produces 5 kW of heat for every 1 kW of electricity at that specific condition. The catch: real UK winters spend most hours well below 7°C, and most installations run flow temperatures higher than 35°C. So the brochure COP is the best case, not the typical case.

How is SCOP different from COP?

SCOP is COP averaged across a full year's typical operating conditions in a defined climate zone (the UK is in the 'Average' climate zone under EN 14825). The calculation weights different outdoor-temperature bins by the number of hours per year they occur, runs the heat pump's performance map at each bin, and produces a single annual efficiency number. For a heat pump rated COP 5.0 at A7/W35, the SCOP at the same W35 flow temp is typically 4.0-4.5 — about 10-20% lower. The reason: the SCOP includes the cold winter hours where COP drops to 2.5-3.0. SCOP is the more useful number for sizing annual running cost.

What is SPF and how do I measure it?

SPF (Seasonal Performance Factor) is what the heat pump actually achieves in your specific home — measured by accumulating kWh of heat output and kWh of electricity input over a full year. Requires two meters: a heat meter (MID Class 2 calibrated, ~£400 fitted) on the flow + return pipework, and an electricity sub-meter on the dedicated heat pump circuit. SPF = (annual kWh heat) ÷ (annual kWh electricity). Real-world UK SPF data from Energy Systems Catapult's Electrification of Heat trial shows median ~2.8 for older retrofits, ~3.4 for newer well-tuned systems. SPF is always lower than SCOP because real homes have non-ideal conditions: under-sized radiators forcing higher flow temps, defrost cycles in winter, poor weather-compensation tuning, cylinder cycling at 60°C for Legionella, etc.

What's a good SCOP to expect in a UK installer quote?

Depends entirely on flow temperature. At 35°C flow (underfloor heating or oversized radiators): SCOP 3.8–4.5 is typical for a quality unit. At 45°C flow (most radiator retrofits): SCOP 3.2–3.8. At 50–55°C flow (small or under-sized radiators): SCOP 2.6–3.2. If a quote gives you SCOP without specifying flow temperature, ask — it's meaningless without the temperature context. MCS-compliant quotes should always include SCOP at the design flow temperature. R290 propane heat pumps tend to top the league at low flow temps; R32 units are solid mid-range; R410A is older refrigerant phasing out.

Why does the brand-A vs brand-B SCOP difference look small?

Because at the same flow temperature and same climate zone, all quality heat pumps from major manufacturers (Daikin, Mitsubishi, Vaillant, Samsung, LG, Bosch, Panasonic) are within a 10-20% SCOP band. Heat pump physics is the same; the marginal gains come from compressor design, refrigerant choice, control sophistication. So a quote showing Daikin SCOP 4.2 vs Vaillant SCOP 4.4 at the same flow temperature reflects roughly £40-£80/year running-cost difference for a typical 3-bed semi — real but small. The bigger swing factor is install quality (flow temperature, weather-comp tuning, sizing) — a good install of a 'worse' brand outperforms a poor install of a 'better' brand by 30%+ regularly.

How does SCOP relate to my electricity bill?

Annual heat demand (kWh) ÷ SCOP = annual electricity consumption (kWh). For a typical UK 3-bed semi at 12,000 kWh heat demand: SCOP 3.5 means 3,430 kWh electricity; SCOP 4.0 means 3,000 kWh; SCOP 4.5 means 2,667 kWh. At a flat 28p/kWh tariff: SCOP 3.5 = £960/year, SCOP 4.5 = £747/year — a £213 difference. On a Cosy-blended 17p/kWh: £583 vs £453, a £130 difference. The SCOP gap matters but is dwarfed by tariff choice (worth £300-£500/year vs flat rate). Conclusion: SCOP is important but tariff selection is more important.

SCOP, COP, SPF explained: heat pump efficiency metrics (UK 2026)

The three numbers, in order of how you encounter them

Most UK homeowners meet these metrics in a specific sequence during the heat-pump journey:

COPon the manufacturer’s brochure and on price-comparison sites. Big number, optimistic, useful for cross-brand comparison only.
SCOPon the MCS installer’s quote. Annual-average number for the UK climate zone. This is the number that drives your running-cost estimate.
SPF on your own meter once the install is commissioned. Real-world performance. Usually lower than the quoted SCOP by ~10-20%.

COP — what the brochure tells you

COP (Coefficient of Performance) is the instantaneous ratio of useful heat output to electricity input at a single defined operating point. The standard reference point is “A7/W35”: 7°C outside air, 35°C flow temperature.

Example: a heat pump rated 8 kW heating capacity with 2 kW electrical input at A7/W35 has a COP of 4.0 — for every 1 kWh of electricity it draws, it produces 4 kWh of heat.

Because both A7 and W35 are mild operating conditions, COP is always the highest efficiency number you’ll see. The brochure may also quote A2/W35 (2°C outside, the typical UK winter point) which is more honest but less flattering; COP at A2/W35 is typically 3.5-4.2 for the same unit that hits 5.0 at A7/W35.

SCOP — what your installer quote should declare

SCOP (Seasonal Coefficient of Performance) integrates COP across a whole year’s typical conditions in a defined climate zone. EN 14825 defines three zones: Warm (e.g. southern Europe), Average (covering the UK), and Cold (e.g. Scandinavia).

The calculation takes the heat pump’s performance map (heat output and electricity input across a grid of outdoor-temperature × flow-temperature combinations) and weights it by the hours per year the UK climate spends at each outdoor-temperature bin. The result is a single annual-average efficiency.

Typical UK 2026 SCOP figures by flow temperature for quality mid-market heat pumps:

W35 (underfloor or oversized rads): SCOP 3.8–4.5
W45 (typical radiator retrofit): SCOP 3.2–3.8
W50 (smaller existing rads): SCOP 2.9–3.4
W55 (under-sized rads, not ideal): SCOP 2.5–3.0

SCOP without flow temperature is meaningless — always check the quote specifies both.

SPF — the truth metric

SPF (Seasonal Performance Factor) is the ONLY number that reflects how your specific heat pump performs in your specific home. SPF = (annual kWh of heat delivered) ÷ (annual kWh of electricity consumed by the heat pump).

To measure SPF you need:

A heat meter on the flow + return pipework between the heat pump and the rest of the system. MID Class 2 calibrated, ~£400 fitted. Most new MCS installs include one as standard.
An electricity sub-meter on the dedicated heat pump circuit. Some units have this built into the indoor controller; others require a separate clamp + display.
12 months of continuous logging — SPF only stabilises across a full heating season.

Real-world UK SPF data from DESNZ-funded trials:

Older retrofits (2015-2019): median SPF 2.6-2.9.
Newer well-tuned installs (2022+): median SPF 3.2-3.6.
Best-in-class examples: SPF 3.8-4.2 (well-insulated, W35 flow, smart tariff).

Why SPF is always lower than SCOP

SCOP assumes a single design flow temperature and well-tuned weather compensation. SPF reflects all the real-world deductions:

Defrost cycles — air-source pumps periodically reverse to clear ice from the outdoor coil. Costs 5-10% of annual output.
Cylinder reheat to 60°C for the weekly Legionella cycle. Forces flow temp above the design point briefly. Costs 1-2% annually.
Suboptimal flow-temperature tuning. Most installers commission conservatively warm. Costs 5-15% until the curve is tuned down.
Cycling and standby losses — small but persistent.

Sum of deductions: SPF typically 10-20% below quoted SCOP. So a SCOP-4.0 quote usually delivers SPF 3.2-3.6 measured.

What this means for your decisions

Three practical implications:

When comparing quotes: compare SCOP at the same flow temperature. A Vaillant W45 SCOP 3.6 quote is roughly equivalent to a Daikin W45 SCOP 3.7 quote — within measurement noise.
When sizing radiators: oversize where you can. Going from W50 to W45 design temperature improves SCOP by ~10%; from W45 to W40 another ~10%. Larger radiators (or low-temp emitters like fan convectors) pay back across the 15-year pump life.
When you have a metered install: check SPF at month 6 and month 12. If it’s 25%+ below the quoted SCOP, ring the installer — usually a weather-comp or sizing tweak fixes it.

The summary

COP is the brochure number; SCOP is the quote number; SPF is the truth number. SCOP varies primarily with flow temperature, not brand — so the most impactful decision is how to size emitters for low flow temps, not which manufacturer you pick. After install, measure SPF and tune the weather-comp curve to close the gap between SCOP and SPF. The combined effect of low flow temp + tuned controls + heat-pump-specific tariff is what makes a heat pump cleanly cheaper to run than gas.

SCOP, COP, SPF explained: UK heat pump efficiency metrics in 2026