Electric resistance heating converts one unit of electricity into exactly one unit of heat and cannot do better. A heat pump moves heat instead of creating it, so one unit of electricity delivers three or four units of heat — a coefficient of performance of 3.5 means 3.5 kWh of heat for every 1 kWh of electricity. That multiplier is the entire economic case, and it is not a fixed number.
What moves it most is flow temperature: the hotter the water the pump has to produce, the harder it works and the lower the COP.
Why does flow temperature dominate?
Because the pump is lifting heat across a temperature difference, and the work required grows with the size of the lift. Producing 35 °C water from a 5 °C outdoor source is a much smaller lift than producing 55 °C from the same source.
| Flow temperature | Typical COP | One unit of electricity gives |
|---|---|---|
| 35 °C | ~4.0 | 4.0 units of heat |
| 45 °C | ~3.2 | 3.2 units |
| 55 °C | ~2.5 | 2.5 units |
| Resistance heating | 1.0 | 1.0 unit |
That is why a heat pump is designed around large emitters. Underfloor heating runs happily at 35 °C; a radiator sized for a gas boiler at 75 °C delivers barely half its rated output at 45 °C, which the Delta T section works through.
Replacing radiators is therefore usually part of the installation rather than an optional upgrade, and skipping it is the most common way an installation underperforms.
How does the running cost compare?
It depends on the ratio between electricity and gas prices as much as on the COP. A COP of 3.5 against electricity costing three and a half times as much per kWh as gas is break-even; better than that ratio and the heat pump wins.
That ratio varies by country and over time, which is why the same technology is obviously cheaper to run in some markets and marginal in others. The electricity cost calculator is where the local numbers go in — the arithmetic is a single division and the inputs are the whole argument.
Standing charges and tariff structure matter too. Heat pumps run for long periods at modest power rather than in short bursts, which suits time-of-use tariffs unusually well.
What is the difference between COP and SCOP?
COP is measured at one operating point; SCOP averages across a heating season including the cold days when the lift is largest and the pump performs worst.
A quoted COP of 4.5 at a mild outdoor temperature and a SCOP of 3.1 across the year is not a contradiction — it is the difference between a spot measurement and an annual average, and the annual figure is the one that determines a bill.
The same distinction applies in reverse for cooling, where the seasonal figure is the honest one and the peak-day figure is what the equipment struggles with.
Why does sizing matter so much more than for a boiler?
Because a heat pump cannot modulate as far down as a gas boiler can, and an oversized unit spends its life cycling on and off. Cycling costs efficiency directly and wears the compressor.
An undersized unit falls back on resistance backup heating on the coldest days, which runs at a COP of 1 and undoes a substantial share of the annual saving in the handful of days it operates.
The window between those two failures is narrower than for a boiler, which is why a proper heat-loss calculation matters here and a rule of thumb does not. Insulating first and sizing afterwards is the order that produces the smaller, cheaper, better-performing system.
Weather compensation is the control feature that makes the low flow temperature workable. It varies the flow temperature with the outdoor temperature, running as cool as the day allows and only lifting it when the weather demands — which keeps the seasonal COP near the top of its range instead of at a fixed compromise.
What about hot water?
It is the one load that needs a high flow temperature regardless, because a cylinder has to reach a temperature that controls legionella. That runs at a poor COP, and it is typically a small share of annual energy in a heated house.
Most systems handle it as a separate, brief high-temperature cycle rather than by running the whole system hot — which is why the space heating flow temperature can stay low even though the cylinder does not.
Questions people ask
Do heat pumps work in cold weather? Yes, with a falling COP. Air-source units continue to extract heat well below freezing; what falls is efficiency, not function.
Is a ground source unit better? The ground is a more stable and warmer source in winter, so the seasonal COP is higher. The installation cost is substantially larger, which is what the comparison usually turns on.
Does it need to run constantly? It works best running steadily at low output rather than being boosted twice a day. That is a different control philosophy from a boiler and a common source of disappointment when the old habits are kept.
How much electricity will it use? Annual heat demand divided by SCOP. The kWh calculator turns that into units and the energy consumption calculator into a yearly figure.
The multiplier is real and it is a variable, and flow temperature is the lever. The radiator size calculator shows what output an emitter gives at a lower flow temperature, and the electricity cost and kWh calculators turn a COP into money.