SCOP, COP and HSPF — heat pump efficiency metrics explained

Last reviewed: 14 May 2026.

In short

COP, SCOP and HSPF are all measures of heat pump efficiency — they describe how much heat you get back for each unit of electricity you put in. COP is a snapshot taken in a single set of lab conditions. SCOP is the figure that matters for UK homeowners: it averages efficiency across a full heating season at four test temperatures, using the European standard BS EN 14825:2022. HSPF is the North American equivalent — and because it is tested in a much warmer US climate, an HSPF figure cannot be directly converted to an EN 14825 SCOP without a 20–30% derating. UK heat pumps carry an ErP energy label (A+++ to G) based on their SCOP. For a Reading install in May 2026, the SCOP figures to look for on a spec sheet sit between 3.9 and 5.0; real-world fleet data from Octopus Cosy installs shows 3.72 across the year, and the independent Heat Pump Monitor dataset averages 3.9. Below 3.0 in service is a signal that something in the design or commissioning is off.

On this page

• The simple version
• COP — efficiency at a single point
• SCOP — the seasonal figure that actually matters
• HSPF — and why it doesn’t translate
• ErP energy labels — A+++ to G
• Declared SCOP vs real-world SPF — the gap
• How SCOP turns into your annual bill
• What this means for homes in Reading
• Related guides

The simple version

A heat pump’s efficiency is the ratio of heat delivered to electricity consumed. If a heat pump produces 3 kWh of heat for every 1 kWh of electricity, its efficiency is 3.0 — sometimes expressed as 300%. That ratio is not a fixed number. It changes constantly with outdoor temperature, with the temperature of the water leaving the heat pump to your radiators, and with how hard the heat pump is being asked to work.

That’s why there are three different metrics. Each one captures the ratio in a different way:

• COP captures it at a single moment, under specific lab conditions.
• SCOP captures it as an annual average, weighted across the seasonal mix of outdoor temperatures a typical UK heating year produces.
• HSPF is the US version of SCOP, calculated under different climate assumptions.

The figure to focus on for a UK install is SCOP — and specifically the SCOP quoted at your install’s expected flow temperature, not the headline 35°C number. Our guide to how heat pumps work covers the underlying physics that makes >100% efficiency possible.

COP — efficiency at a single point

COP — the Coefficient of Performance — is the simplest measure. It’s the ratio of heat output to electricity input at one specific moment:

COP = heat output (kW) ÷ electrical input (kW)

A heat pump producing 6 kW of heat using 2 kW of electricity has a COP of 3.0.

The number depends on three things that change continuously:

• Outdoor temperature. The colder the air outside, the more “lift” the heat pump must achieve, and the lower the COP. A unit running COP 4.5 on a mild +7°C day might drop to COP 2.5 at −7°C.
• Flow temperature. The temperature of the water leaving the heat pump to your radiators. Higher flow temperatures mean lower COP. The same unit running COP 3.8 at a 35°C flow temperature might run COP 2.8 at 55°C.
• How hard it’s working. Modern heat pumps use variable-speed inverters that modulate their output. At very low part-load — running at a fraction of rated capacity — the electronics and standby overheads eat a bigger share of the input, dragging COP down.

A COP figure quoted in isolation — “this heat pump has a COP of 4.5” — isn’t very useful unless you know the conditions. Manufacturer spec sheets typically quote COP at A7/W35 (7°C ambient air, 35°C flow water), which produces a flattering number that bears little relation to a January morning in Reading at −2°C ambient with the radiators running at 48°C.

COP is best understood as a snapshot. The figure that integrates all the snapshots into a useful seasonal average is SCOP.

SCOP — the seasonal figure that actually matters

SCOP — the Seasonal Coefficient of Performance — is the metric designed to predict real-world annual performance. It’s calculated under BS EN 14825:2022, the European and British standard that governs heat pump efficiency testing.

The calculation works in bins:

1. The heating season is divided into a number of hours at different outdoor temperatures (the “bins”), based on a reference climate. The UK uses the EN 14825 “Average” climate profile (based on Strasbourg) by default for MCS purposes.
2. The heat pump is tested at four specific outdoor temperatures: +12°C, +7°C, +2°C, and −7°C, plus the bivalent point (where the heat pump’s maximum output equals the building’s heat demand) and the temperature operation limit (below which it cannot operate without electric backup).
3. COP at each test point is weighted by the bin-hours at that temperature and aggregated into a single seasonal figure.

The result is a number that captures expected annual performance across the full mix of weather the climate produces.

There is one further nuance: SCOP is quoted at a specific flow temperature, and there are typically two figures on a spec sheet:

• SCOP at 35°C flow — the headline figure, achievable in well-designed new-build or fully retrofitted properties where the radiators or underfloor heating are sized for low-temperature operation. This is the number you’ll see on most product pages.
• SCOP at 55°C flow — the more relevant figure for retrofit installs that haven’t upgraded their emitters. It will be 0.5 to 0.8 lower than the 35°C number, because the compressor is doing more work at the higher lift.

When you compare heat pumps, compare them at the flow temperature your install will actually run at. The 35°C figure is meaningful if your install reaches 35°C; it is misleading if your install runs at 50°C. Our guide to heat loss surveys explains how that flow temperature gets decided.

HSPF — and why it doesn’t translate

HSPF — the Heating Seasonal Performance Factor — is the North American equivalent of SCOP. It is calculated under the AHRI 210/240 standard rather than EN 14825, and it has two structural differences worth understanding.

Different units. HSPF is measured in BTU/Wh (British Thermal Units per watt-hour). SCOP is measured in W/W (or kWh/kWh) — dimensionless. The arithmetic conversion is:

SCOP = HSPF ÷ 3.412
HSPF = SCOP × 3.412

A heat pump with HSPF 10 has an arithmetic SCOP equivalent of 2.93.

Different climate. HSPF is rated against US DOE Region IV climate data, which is materially warmer than the EN 14825 European Average profile. A heat pump rated HSPF 10 in Region IV would deliver real-world performance noticeably worse than its arithmetic SCOP 2.93 in a UK climate — because the colder winter bins in EN 14825’s calculation weight the result toward the colder test points where COP is lowest. The accepted derating sits in the 20–30% range.

The practical implication for UK buyers: HSPF is not a like-for-like substitute for SCOP. A US-imported heat pump, or a spec sheet quoting only HSPF, cannot be directly compared to a UK EN 14825 SCOP without applying the climate derating. MCS submission and Boiler Upgrade Scheme grant eligibility both require an EN 14825 SCOP.

A newer US metric — HSPF2 — was introduced in 2023 alongside SEER2, using revised test procedures that produce numerically lower values than HSPF. HSPF and HSPF2 are not interchangeable. If you’re cross-checking a US product spec, look for which version it quotes.

ErP energy labels — A+++ to G

UK heat pumps sold for residential use carry an ErP energy label under the Ecodesign Regulation 813/2013 (retained in UK law post-Brexit). The label rates the heat pump’s seasonal space-heating energy efficiency on a scale from G (least efficient) to A+++ (most efficient).

The thresholds, at the 35°C flow temperature where most labels are quoted:

ErP rating	Seasonal efficiency (ηs,h)	Approximate SCOP
A+++	≥175%	≥4.5
A++	≥150%	≥3.85
A+	≥125%	≥3.2
A	≥115%	≥2.95
B	≥107%	≥2.75
C	≥100%	≥2.55

(At a 55°C flow temperature the thresholds are correspondingly lower, since the same unit delivers a lower SCOP at higher flow.)

Most modern UK air source heat pumps sold for residential install fall in the A++ to A+++ range at 35°C flow. Within that band there is still material spread — see our brand comparison guide for the unit-by-unit detail.

One point worth flagging: the EU rescaled energy labels for white goods (fridges, washing machines, dishwashers) in 2021 to remove the A+/A++/A+++ tier — those products now use a flat A-G scale. Heat pumps were not included in that rescale and still carry the A+++ tier. Some online sources conflate the two; for heat pumps specifically, the A+++ to G range remains current as of May 2026.

Declared SCOP vs real-world SPF — the gap

The SCOP quoted on a spec sheet is a laboratory figure. The real-world figure delivered in service is usually called SPF — Seasonal Performance Factor — and it is measured by metering actual heat output and actual electricity consumption across a full year of real operation.

Three datasets give a useful UK benchmark as of May 2026:

Octopus Cosy fleet performance. Octopus publish a live fleet-wide SPF dashboard for all their installed Cosy heat pumps. The most recent full-year figure (1 May 2025 to 30 April 2026) is SPF 3.72 for combined space heating and hot water. This is real-world data from thousands of installed units, not a manufacturer-declared SCOP.

Heat Pump Monitor. An independent open-source dataset reporting real-time performance from hundreds of UK heat pump installs. The average SPF sits at approximately 3.9.

Electrification of Heat trial. The government-backed pilot ran 750 heat pump installs across a range of UK property types between 2020 and 2022 and reported a fleet SPF of 2.8. This is the figure embedded in DESNZ modelling assumptions, and it is materially lower than the more recent Octopus and Heat Pump Monitor figures.

The gap between the trial’s 2.8 and the current 3.7–3.9 reflects four things:

• Better installer design. Lower flow temperatures (45–50°C targets rather than 55°C defaults), better radiator sizing, more rigorous compliance with MIS 3005-D — the MCS Heat Pump Design Standard, currently at V3.0 since 5 December 2025.
• Better unit selection. Specifying on declared SCOP at the actual operating flow temperature, not on the flattering 35°C headline number.
• Better commissioning. Weather compensation properly tuned at handover; flow setpoints reviewed in service; controls left in the modes the manufacturer designed.
• Heat-pump-friendly tariffs. Lower £/kWh during operating hours when the heat pump is working hardest, shifting the cost-equivalence threshold.

The takeaway: a quoted SCOP figure is a design baseline, not a guarantee. Real-world delivery in a well-installed Reading property in 2026 sits in the 3.5–4.2 range. Below 3.0 in service is a signal that something in the design or commissioning is wrong — most often a default 55°C flow temperature where 45°C would have been workable with modest radiator upgrades.

How SCOP turns into your annual bill

SCOP maps directly to running cost through a simple relationship:

Annual electricity for heating (kWh) = Annual heat demand (kWh) ÷ SCOP

A Reading 3-bed semi using 15,000 kWh of heating per year — typical for a cavity-wall property in Tilehurst or Earley after fabric upgrades — delivered at SCOP 3.5 consumes 4,286 kWh of electricity. At 27p/kWh on a standard variable tariff (the May 2026 cap-tracking range), that’s £1,157 a year for heating. The same 15,000 kWh delivered at SCOP 4.0 consumes 3,750 kWh, costing £1,013 — a £144 annual saving from the 0.5 SCOP improvement.

This is the lever that makes heat-pump-specific tariffs economically powerful when they are available: a tariff delivering a meaningful discount during the heat pump’s main operating hours can drop the effective £/kWh by 5–10p, reducing the annual figure further. Our running cost guide walks through the tariff and bill arithmetic in detail.

The SCOP-to-running-cost mapping is also why the design decisions made at the heat-loss survey stage are so consequential. A survey defaulting to 55°C flow to avoid radiator upgrades may save £1,000–£2,000 on the install but cost 0.5–0.8 SCOP for the 15–20 year life of the unit — typically £150–£300 a year in running cost lost, or £3,000–£6,000 over the asset’s life. That trade-off is worth making with eyes open rather than by default.

What this means for homes in Reading

The Reading housing stock makes SCOP outcomes vary substantially by neighbourhood — not because the climate is different (the Thames Valley CIBSE design external temperature is −3.0°C uniformly across the Reading area), but because the fabric determines how hot the radiators need to run.

In central Reading and lower Caversham — Victorian and Edwardian terrace stock with solid 9” brick walls and often older glazing — heat losses are typically high, and emitter sizes are often constrained by available wall space. The flow temperature defaults end up nearer 50–55°C without internal wall insulation, and SCOPs in service typically come in at 3.0–3.5. With internal insulation and emitter upgrades, central Reading installs can pull into the 3.5–3.9 range.

In Tilehurst, Earley, Whitley and the eastern wedge — inter-war and post-war semis with cavity walls — filled cavities and modest radiator upgrades commonly support 45–50°C flow temperatures and SCOPs in the 3.6–4.0 band.

In Lower Earley, Woodley, and the modern estates — 1980s+ construction with insulated cavities and modern glazing — flow temperatures of 40–45°C are routinely achievable with minimal radiator changes, and SCOPs in the 4.0–4.4 range are common. These are the properties where the highest real-world efficiency figures land.

Caversham Heights, Caversham Park, and parts of central Reading contain conservation areas and listed properties where fabric upgrades face planning constraints. Heat losses cannot always be reduced, which pushes flow temperatures up and SCOPs down. A SCOP of 3.0–3.3 is more typical here, and is often acceptable given the property constraints.

The pattern matters because the SCOP figure on a spec sheet is a starting point — what you get in service depends as much on the property and the design choices as on the unit you bought.

Related guides

• How does an air source heat pump work? The complete explanation — the underlying physics behind heat pump efficiency
• Heat pump running cost in the UK — what to expect in 2026 — where SCOP turns into annual £
• Heat loss surveys for heat pumps — what they are, what to expect — the design stage that fixes your SCOP for the life of the install
• Best heat pump brands UK 2026 — eight brands compared — brand-by-brand SCOP figures at different flow temperatures

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Ready to design a heat pump system that actually delivers its declared SCOP? Our MCS-certified engineers design every install at the lowest workable flow temperature for the property, with the radiator and emitter changes that get the SCOP into the 3.7–4.2 range.

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