COP stands for Coefficient of Performance, and it’s simply the ratio of heat a heat pump delivers to the electricity it consumes to produce it. A COP of 3 means that for every 1kW of electricity you put in, you get 3kW of heat out of your radiators or underfloor heating. It’s the single number that tells you how efficiently your heat pump is converting electricity into warmth. Unlike a gas boiler, which can never produce more heat energy than the fuel energy it burns, a heat pump routinely delivers two, three or four times more heat energy than the electrical energy it uses, because it’s extracting additional free heat from the air outside.
That efficiency isn’t fixed, though. It moves around depending on the weather, how you’ve set your system up, and how well it’s been installed and maintained. Understanding what drives COP up and down, and how to check your own system’s real-world figure, is one of the most useful things a heat pump owner can learn, both for keeping running costs down and for spotting problems early.
What’s the difference between rated COP and SCOP?
Rated COP is a single lab-measured figure taken under one fixed set of conditions, while SCOP is a seasonal average that reflects real-world UK weather, which is why SCOP is the more useful number for predicting your actual running costs. The rated COP shown on a manufacturer’s spec sheet or a quote is almost always measured at a single test point; the most commonly quoted one in Europe is A7/W35, meaning 7°C outdoor air temperature and a 35°C flow temperature leaving the heat pump. Under those conditions, a good modern air source heat pump might achieve a COP of 4 or higher.
The problem with rated COP is that it describes one moment, not a British winter. Your heat pump doesn’t run at a constant 7°C outside with a 35°C flow temperature all year. It runs through freezing January nights, mild October afternoons, and everything in between, often with the flow temperature adjusted to suit whatever weather compensation curve or thermostat setting you’ve chosen.
This is where SCOP, Seasonal Coefficient of Performance, becomes the far more meaningful figure for a UK homeowner. SCOP is a weighted average of performance across an entire heating season, calculated against a representative climate profile, so it captures the colder days when the heat pump is working harder and less efficiently, alongside the milder days when it’s coasting. A heat pump with a rated COP of 4.5 at A7/W35 might have a real-world SCOP closer to 3.2–3.8 once a full UK winter is factored in. Neither number is wrong. They’re just answering different questions. Rated COP tells you the ceiling. SCOP tells you what you can actually expect to pay for over a season.
When comparing heat pumps, or trying to work out why your running costs look the way they do, always look for the SCOP figure rather than the headline lab COP.
How does outdoor temperature affect COP?
COP falls as the outdoor temperature drops, because the colder the air, the less ambient heat is available to extract and the harder the compressor has to work to bridge the gap between outdoor and indoor temperatures. A heat pump works by extracting heat from the outside air (or, less commonly in the UK, from the ground or water) and concentrating it to a useful temperature using a refrigerant cycle and a compressor, and that extra compressor effort costs electricity.
The table below illustrates the general pattern you’d expect from a typical modern air source heat pump running at a fixed flow temperature. Exact figures vary by model, refrigerant, and system design, but the downward trend as temperatures fall is consistent across virtually all air source systems.
| Outdoor temperature | Typical COP (illustrative) | What’s happening |
|---|---|---|
| 10°C | ~4.0–4.5 | Plenty of ambient heat available; compressor works lightly |
| 7°C | ~3.8–4.2 | Close to standard rated test conditions |
| 2°C | ~3.0–3.4 | Compressor working noticeably harder |
| -2°C | ~2.4–2.8 | Cold snap conditions; efficiency drops further |
| -7°C | ~2.0–2.3 | Near the lower end of typical UK design temperatures |
This is a normal characteristic of heat pump technology, not a fault, but it’s exactly why SCOP, which averages across a season rather than quoting a single mild-weather figure, gives you a truer picture of what to expect from your energy bills.
How does flow temperature affect COP?
The lower the flow temperature, the temperature of the water leaving the heat pump on its way to your radiators or underfloor pipework, the higher your COP, because the compressor has less work to do to lift the refrigerant to that temperature. Flow temperature has just as much influence on COP as outdoor temperature, and it’s the one factor you and your installer have direct control over.
This is the reason heat pumps are so well suited to underfloor heating, which can keep a room comfortable at flow temperatures as low as 30–35°C thanks to the large surface area involved, and why oversized radiators (bigger than you’d fit with a gas boiler) are often specified for heat pump retrofits. They can push out the same heat output at a lower water temperature, keeping the flow temperature (and therefore the COP) more favourable.
| Flow temperature | Typical COP (illustrative) | Typical emitter type |
|---|---|---|
| 30–35°C | ~4.0–4.5 | Underfloor heating |
| 40°C | ~3.5–4.0 | Oversized/low-temperature radiators |
| 45–50°C | ~3.0–3.5 | Standard-sized radiators, mild weather compensation |
| 55°C | ~2.5–3.0 | Undersized radiators, higher flow temp needed to compensate |
| 60°C+ | ~2.0–2.5 | Flow temperature set unnecessarily high, or system struggling to heat the house otherwise |
If your installer has set up weather compensation properly, your flow temperature should automatically drop on milder days and rise only when it’s needed on colder ones, rather than sitting at a fixed high temperature all year round regardless of conditions.
What counts as a good COP in UK conditions?
As a general guide, a SCOP of 3.5 or above is considered good in UK conditions, 2.5–3.5 is acceptable, and anything persistently below 2.5 is worth investigating, though there’s no single number that applies to every property, emitter type and region, so treat this as guidance rather than a strict pass/fail test. A system running a well-insulated new build with underfloor heating in the South East will naturally perform differently to an older solid-wall property with radiators in the Scottish Highlands.
| SCOP range | Rating | What it suggests |
|---|---|---|
| 3.5 and above | Good | System is well matched to the property and running efficiently |
| 2.5 – 3.5 | Acceptable | Reasonable performance; worth reviewing settings for possible gains |
| Below 2.5 | Investigate | Something is likely holding performance back. Worth a proper check |
A SCOP sitting persistently below 2.5 doesn’t automatically mean something is broken, but it’s a signal worth taking seriously. It’s the kind of figure that, on its own, tells you to look closer rather than panic. The next step is finding out why.
How can you check your own system’s real-world COP?
How do you check COP using your system’s built-in monitoring?
You check COP using built-in monitoring by looking at the electricity-consumed and heat-delivered figures shown in your heat pump’s controller, companion app, or add-on energy meter. Most systems installed in the UK over the last few years include one of these. Some will calculate and display a running COP or SCOP figure directly, and this is by far the easiest way to keep an eye on performance; check it periodically rather than obsessively, since day-to-day COP will naturally bounce around with the weather.
How do you work out COP from meter readings?
You work out COP from meter readings by dividing heat delivered by electricity consumed over the same period (a full week is more reliable than a single day), using two figures if your system doesn’t have built-in heat metering:
- Electricity consumed by the heat pump, read from a dedicated electricity meter or smart plug/monitor on the heat pump’s circuit
- Heat delivered, either from a heat meter fitted to the system (increasingly common on newer installations) or estimated from your usual heat demand if no heat meter is fitted
It’s not laboratory-precise without a proper heat meter, but tracked consistently over weeks and months, it’s more than good enough to spot trends.
When should you bring in an installer or engineer?
You should bring in an installer or engineer if you don’t have any form of monitoring, or if the numbers you’re seeing look consistently poor and you can’t explain why. A qualified engineer can attach diagnostic equipment, check refrigerant charge and pressures, review your controller’s flow temperature and weather compensation settings, and compare actual performance against what the system was designed to achieve.
What causes poor COP?
Poor COP is usually caused by one of six factors: incorrect sizing, too-high flow temperature, a refrigerant leak, poor building insulation, installation faults, or a dirty outdoor unit. When a heat pump’s real-world COP is consistently lower than expected, the cause falls into one of these categories:
- Incorrect sizing. An undersized heat pump has to run harder and longer to meet heat demand, while an oversized one can short-cycle, both of which hurt efficiency.
- Flow temperature set too high. Often left over from a cautious initial commissioning, or never adjusted after installation. This alone can be the difference between a good and a poor COP.
- Refrigerant leak or low charge. A slow leak reduces the system’s ability to extract and transfer heat effectively, and typically worsens gradually over time.
- Poor building insulation or heat loss. The heat pump isn’t the problem here. The house is demanding more heat than it should, forcing higher flow temperatures and longer run times to compensate.
- Installation faults. Poor pipework design, incorrect commissioning, air in the system, or a badly configured weather compensation curve can all quietly suppress performance from day one.
- Dirty or obstructed outdoor unit. A blocked, dirty, or poorly sited outdoor unit (too close to a wall, surrounded by debris, or in a spot with poor airflow) restricts the heat pump’s ability to draw in air efficiently.
Why does tracking COP year-on-year matter?
Tracking COP year-on-year matters because it reveals gradual declines (a slowly dropping refrigerant charge, a gradually fouling outdoor unit, or a drifted controller setting) as an early warning sign, well before the system develops an obvious fault or breaks down. A single low reading on a freezing week isn’t cause for concern on its own; what matters is the trend. If you compare this winter’s average COP against last winter’s for similar outdoor conditions and see a meaningful decline, that’s the signal to act on. Catching that early usually means a straightforward service visit rather than an emergency callout, and it keeps your running costs where they should be in the meantime.
How do you get started monitoring your own COP?
- Check whether your heat pump’s app or controller already displays energy consumed and heat delivered. Most modern systems do.
- If it doesn’t, look at fitting a heat meter alongside an electricity monitor so you can calculate COP from readings yourself.
- Take readings over comparable periods (weekly is a good balance) rather than single days, since daily figures will vary a lot with the weather.
- Keep a simple record so you can compare the same time of year, year-on-year, rather than comparing a mild week to a cold one.
- If your figures sit persistently below 2.5, or you notice a clear downward trend compared to previous seasons, get an engineer to check flow temperature settings, refrigerant charge, and the outdoor unit before it develops into a bigger problem.
A heat pump that’s sized correctly, commissioned properly, and running at sensible flow temperatures should reward you with a strong SCOP and low running costs for years. Keeping half an eye on that COP figure, rather than only noticing when something goes wrong, is one of the simplest ways to make sure it stays that way.
