How a heat pump works: a simple explanation
A heat pump does not make heat by burning fuel. It takes heat from outdoor air, the ground or water and moves it into the house. Electricity drives the compressor that makes this transfer possible, so the heat pump delivers more heat than the electricity it uses.
- From 1 kWh of electricity in testing
- 2.13 to 4.55 kWh of heat
- In 49 monitored homes in Germany in 2024
- 3.4 kWh of heat on average
Testing: one air-to-water model according to Panasonic's data, from frost with hot water to mild weather with lukewarm water. Real homes: 49 monitored air-to-water heat pumps in Germany (Fraunhofer ISE, WP-QS im Bestand project). We explain both figures below.
Moving heat in four steps
Even cold winter air contains heat; its temperature is just too low to heat a house. A heat pump therefore “pumps” that heat up to a higher temperature, much as a fridge moves heat from inside to outside, only in the opposite direction. It does this with a substance called a refrigerant, which keeps changing state in a closed circuit (SIEA):
- The evaporator absorbs heat from the surroundings. The refrigerant evaporates in it at a low temperature, so it can take heat even from cold air, ground or water.
- The compressor compresses the refrigerant vapour. Compression heats it to a temperature suitable for heating. Electricity drives this step.
- The condenser releases the heat to the heating water or the room air. The refrigerant cools and turns back into a liquid.
- The expansion valve lowers the refrigerant's pressure. This cools the refrigerant below the temperature of its surroundings again, and it returns to the evaporator.
Electricity and the heat gained
The heat a heat pump delivers to the house has two sources: heat from the surroundings and the electricity used by the compressor. You do not pay for the ambient heat itself. Operation does, however, need electricity for the compressor and for auxiliary equipment such as a fan or circulation pump.
Example: if a heat pump reached a COP of 3 under certain conditions, it would deliver 3 kWh of heat from 1 kWh of electricity and 2 kWh of ambient heat. About 67% of the heat would therefore come from the surroundings. The figure 3 is chosen for simplicity; it does not apply to a particular model or to a whole year.
kW and kWh are not the same
A kilowatt (kW) is power: how much heat the heat pump delivers at a given moment. It is used to match the heat pump to the house's heat loss. A kilowatt-hour (kWh) is an amount of energy over time. Electricity on a bill and heat used over a year are measured in kWh. A 9 kW heat pump running at full output for 1 hour delivers 9 kWh of heat.
What COP, SCOP and SPF mean
All three figures tell you how much heat the heat pump delivers from 1 kWh of electricity. They differ in the conditions and the period over which they were measured.
- COP
- An instantaneous ratio at one test condition, for example 7 °C outdoors and 35 °C heating water. The manufacturer states it (SIEA). It describes one moment, not the whole year.
- SCOP
- A seasonal ratio for the whole heating season in a reference climate, calculated under EU rules (Regulation 813/2013). It is better for comparing models, but it still applies to test conditions, not to your house.
- SPF
- A ratio measured in a real house, usually over a year. It depends on what the measurement includes, such as hot water or an electric immersion heater (SIEA).
For one 9 kW air-to-water model, it looks like this:
- COP at 7 °C outdoors and 35 °C water: 4.55,
- SCOP at 35 °C water in an average climate: 4.84; in a colder climate 4.31.
A particular house's SPF is established from its operation. For example, the German study Fraunhofer ISE, WP-QS im Bestand project evaluated 49 air-to-water heat pumps in occupied homes, including older ones, in 2024. Their SPF ranged from 2.6 to 4.9, with an average of 3.4 (Fraunhofer ISE, checked 20 October 2026). The measurement covered space heating and hot water, with electricity for the compressor, controls, electric immersion heater and source pumps or fan. These are different homes, a different climate and different measurement boundaries from SCOP, so do not compare the figures directly. They do show, however, that the same technology gives very different results in different homes.
Why source and water temperatures matter
The bigger the difference between the source temperature and the heating-water temperature, the more work the compressor must do and the more electricity it uses. The same heat pump therefore achieves different results in frost than in autumn, and with underfloor heating than with small radiators (SIEA).
That is why manufacturers state values at several test points. A7/W35 means 7 °C outdoors (A for air) and 35 °C heating water (W for water). The same model as above:
- Outdoor temperature: at −7 °C instead of 7 °C, COP fell from 4.55 to 2.8 and output from 9 to 7 kW.
- Water temperature: with 55 °C water instead of 35 °C, COP fell from 4.55 to 3.03.
- Both at once: in frost with 55 °C water, the heat pump delivered only 2.13 kWh of heat from 1 kWh of electricity.
For a household, the main lesson is this: the lower the water temperature a house needs to stay warm, the less electricity the heat pump uses. That is why it suits underfloor heating or large radiators best. We explain how small radiators fit in our article on heat pumps for an older house.
How the types differ
The principle is the same in every heat pump. The types differ in where they take heat from and where they deliver it. The name tells you in that order: air-to-water takes heat from air and heats the water in the heating system.
- Air-to-water
- Takes heat from outdoor air and heats water for radiators or underfloor heating, as well as domestic hot water.
- Air-to-air
- Heats and cools rooms through wall-mounted indoor units, like air conditioning. It does not heat water.
- Ground-source
- Extracts heat from the ground through a borehole or garden collector. Ground works make up a substantial part of the cost.
- Water-source
- Extracts heat from groundwater in wells. It needs enough suitable water and the relevant permissions.
The ground and groundwater have a more stable temperature in winter than outdoor air, so the compressor has less work to do. In the German study mentioned above, 13 ground-source heat pumps achieved an average SPF of 4.3 compared with 3.4 for air (final results). The sample was small and this is not a forecast for Slovakia. A borehole or collector also makes up a large part of the price, so a higher ratio does not by itself mean ground-source is better value.
Monobloc and split are not further types but two designs of the same air-to-water heat pump. In a monobloc the whole refrigerant circuit is in the outdoor unit; in a split the refrigerant runs to the indoor unit (SIEA). We compare the differences on the monobloc heat pump page.
Common questions
Does it produce heat below freezing?
Yes. Even freezing air contains heat that the refrigerant can absorb. Output and COP change with the outdoor temperature and the specific model; in frost a heat pump generally works at a lower COP. The model in the table above delivered 7 kW with a COP of 2.8 at −7 °C and 35 °C water. When choosing, ask about output at the lowest design temperature for your area and about the lowest outdoor temperature at which the specific model still operates. The manufacturer states this for each model.
Does a COP of 4 mean four times the savings over a year?
No. A COP of 4 means that at one test condition the heat pump delivers 4 kWh of heat from 1 kWh of electricity. Conditions change during the year, so the annual ratio is usually different. Savings also depend on what you heat with today, electricity and fuel prices, and how much heat the house uses. You can estimate annual consumption in our article on heat-pump electricity consumption. A cost comparison with gas is in heat pump vs gas boiler.
What is the difference between kW and kWh?
kW is power: how much heat the heat pump delivers at a given moment. kWh is an amount of energy over time, such as the electricity on a bill. A 9 kW heat pump running at full output for 1 hour delivers 9 kWh of heat. If its COP is 3 at that point, it uses 3 kWh of electricity.
Sources
The principle and the terms COP and SPF are based on the SIEA guide to choosing a heat pump (in Slovak), and the SCOP definition on EU Regulation 813/2013. The Panasonic model's output, COP and SCOP come from the manufacturer's technical data at test conditions, not from measurement in a real house. The SPF figures come from measurements in occupied homes in Germany and do not predict results in Slovakia. The diagrams are our own and simplified. We do not sell or install heat pumps, and manufacturers do not pay us to mention their models.
- SIEA: choosing a heat pump (Slovak) — the principle, COP, SPF and differences between types.
- EU Regulation 813/2013, Annexes I and III — SCOP definition and reference conditions.
- Panasonic: WH-WDG09LE5 — output, COP and SCOP of the stated combination, checked 20 October 2026.
- Fraunhofer ISE: WP-QS im Bestand — measurements in 2024, checked 20 October 2026.