Energy dataLast updated: 390 monitored systems, twelve months to date

What Heat Pumps Actually Achieve in Real British Homes

Contents

Almost everything written about heat pump efficiency is either a manufacturer’s laboratory rating or somebody’s anecdote. This page is neither. It is what several hundred British households have measured in their own homes over a year and chosen to publish.

What the number means

A heat pump’s coefficient of performance is how many units of heat it delivers for each unit of electricity it uses. A COP of 4 means four kilowatt-hours of heat from one kilowatt-hour of electricity — which sounds impossible until you remember the machine is not making heat, it is moving heat that was already outside. A gas boiler cannot exceed 1, because burning the fuel is all it does.

That ratio is what decides whether a heat pump is cheaper to run than the boiler it replaced, once you set it against the price of electricity versus gas.

What these homes actually achieve

Grouped by what the meters actually covered, because that changes the answer more than anything about the machines does.

Whole system metered

The fullest arrangement: what went in and what came out, both measured.

Everything together: 3.97middle half 3.57–4.28325 systems
Heating the rooms: 4.25middle half 3.86–4.62215 systems
Heating the hot water: 3.07middle half 2.65–3.44200 systems

Most of the system metered

Closer to complete, but still not everything the system draws.

Everything together: 3.73middle half 3.33–4.2646 systems
Heating the rooms: 4.15middle half 3.66–4.5726 systems
Heating the hot water: 3.17middle half 2.78–3.4825 systems

Partial metering

Some electricity the system uses falls outside what was measured, so these figures read higher than the whole system would.

Everything together: 3.54middle half 3.29–4.1318 systems
Heating the rooms: 3.85middle half 3.57–4.2411 systems
Heating the hot water: 3.15middle half 2.61–3.2810 systems

The colder the water, the better it does

Median across everything, for whole-system-metered installations only. Grouping by flow temperature across different metering arrangements would confuse the two effects, so this is one boundary.

Flow temperature 35 to 45°C: 3.9146 systems
Flow temperature Under 35°C: 4.0893 systems

781 systems publish their monitoring; 390 are used in the figures above. 50 are excluded because the publisher flags something about how they were measured, and rows failing a quality check are excluded too. These are households who chose to publish their own data, not a representative sample of British heat pumps.

Why there is no single figure here

You will have noticed there is no headline number, and that is deliberate. The monitored systems did not all measure the same thing. Some metered the whole system — every unit of electricity in, every unit of heat out. Others metered most of it, and some only part.

A COP that leaves out some of the electricity a system draws reads higher than one that counts all of it, so the arrangements are not comparable. Compare the three groups above and the gap between them is wider than most of the differences anyone would want to draw from this data. Averaging across them would produce the tidy single figure everybody wants, and it would be an artefact of how the meters were fitted rather than a fact about heat pumps.

Hot water is where they struggle

The single most useful thing in these figures is the gap between heating a room and heating a tank. Space heating comfortably clears a COP of 4 in the best-metered group; hot water sits near 3.

The reason is physics rather than fault. A cylinder needs water at 50°C or more to be safe and useful, and a heat pump becomes less efficient the hotter it has to push. Radiators and underfloor heating can be run far cooler. That gap is why a heat pump’s whole-year figure always lands below its space heating figure, and why a system that heats water efficiently — a well-insulated cylinder, a sensible reheat schedule — is worth more than it looks.

The one thing that changes the answer most

Flow temperature is the water temperature the system sends round the house, and it is the design decision that most affects what a heat pump costs to run. The monitored homes running under 35°C achieve a clearly better median than those between 35 and 45°C.

Running cooler is not something you do afterwards by turning a dial. It comes from having enough radiator or underfloor area to give off the heat at a lower temperature, which is decided when the system is specified. That is the same point our deployment page makes about insulation: a well-specified heat pump in a poorly prepared house is the standard route to disappointment.

What the electricity itself costs is the other half of the sum, and both suppliers price heat pumps separately from ordinary use — see Cosy Octopus and EDF Heat Pump Tracker. If a grant is part of your arithmetic, the Boiler Upgrade Scheme is worth more than its headline figure for a home off the gas grid.

Whose homes these are

These are households who monitor their own heat pumps and publish the results. That is the strength of the data and its limitation in the same sentence: it is genuine measurement over a year rather than a rating from a test rig, and it selects for people interested enough to instrument their heating.

So read these as what a heat pump can do in a British house, not what an average installation does. The gap between the two is mostly specification and commissioning, which is the encouraging part — it means the good outcomes here are reproducible rather than lucky.

Systems the publisher flags as having a measurement problem are excluded rather than quietly included, as are readings that fail a plausibility check. No household location is stored anywhere in this data.

Frequently asked questions

What is a COP?

The coefficient of performance: units of heat delivered for each unit of electricity used. A COP of 4 means four kilowatt-hours of heat from one kilowatt-hour of electricity. A gas boiler cannot exceed 1, because it is burning the fuel rather than moving heat that is already outside.

Why is there no overall average on this page?

Because the monitored systems did not all measure the same thing. Some metered the whole system, others only part of it, and a COP that excludes some of the electricity a system draws reads higher than one that includes it. Averaging across those arrangements produces the single number everyone wants and it does not mean anything, so the figures stay grouped by what was actually measured.

Do these figures apply to my house?

Not directly. These are households who chose to publish their monitoring, which selects for people who are interested in getting it right — well-specified installations, often carefully commissioned. Read them as what a heat pump can achieve in a British home rather than what an average installation does.

Why is hot water so much worse than heating?

Because a cylinder needs water at 50°C or more, and a heat pump gets less efficient the hotter it has to push. Space heating through radiators or underfloor can run much cooler. The gap in this data is large and consistent, and it is the reason a heat pump’s annual figure is always lower than its space heating figure alone.

What flow temperature should I aim for?

Lower than a gas boiler would use, which usually means larger radiators or underfloor heating rather than a hotter system. The monitored homes running under 35°C achieve a noticeably better median than those between 35 and 45°C. That is the design decision that most affects running cost, and it is made when the system is specified, not afterwards.