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Glossary · Energy industry & energy transition

COP, coefficient of performance

COP coefficient of performance heat pump Seasonal performance factor Efficiency temperature lift

The COP, short for "Coefficient of Performance", describes for a heat pump the ratio between the heat output delivered and the electrical power consumed in a given operating state. In German this value is usually called Leistungszahl. A COP of 3 means: under the stated conditions, the system delivers 3 kilowatts of heat output per kilowatt of electrical power. The additional 2 kilowatts do not come from the electricity grid. They come from ambient heat that the system extracts from outdoor air, the ground or groundwater and raises to a higher temperature level.

The COP has no unit because power is divided by power. It is neither an annual value nor a statement about the electricity bill. It describes an instantaneous value or a test value under defined conditions, and its significance depends on those conditions. An air-to-water heat pump can achieve a high coefficient of performance with mild outdoor air and a low flow temperature. In frost and at a high flow temperature it performs considerably worse. A COP value without the temperatures stated is therefore hardly suitable for comparison.

Test points and standards

Manufacturers usually state the COP at standardised test points according to the European test standard EN 14511. The abbreviations name first the heat source, then the flow temperature of the heating water: A stands for outdoor air (Air), B for brine and W for water. A7/W35 means 7 degrees Celsius outdoor air and 35 degrees Celsius flow temperature. A2/W35, A-7/W35 and A-7/W55 represent colder conditions or higher flow temperatures. For brine-to-water heat pumps B0/W35 is customary, for water-to-water heat pumps W10/W35. At the standard rated test point, the test standard also specifies a spread of 5 kelvin between flow and return. Which auxiliary units are included in the electrical power consumption is determined by the respective test standard. Data sheets from different origins are therefore comparable only if the test point and standard match.

How strongly the value depends on the test point is shown by a calculation example from the specialist literature on VDI 4650. For an unregulated air-to-water heat pump, coefficients of performance of 4.36 at A7/W35, 3.64 at A2/W35 and 2.95 at A-7/W35 are assumed there. A unit therefore does not have a single COP but an entire characteristic curve. At A2/W35 one should also check whether the measurement includes defrost cycles. With outdoor air around freezing point, the evaporator ices up, and defrosting costs energy (see defrost cycle).

In EU ecodesign law for space heaters and combination heaters up to 400 kilowatts rated heat output (Regulation (EU) No 813/2013), several terms are explicitly defined. The rated coefficient of performance COPrated is the declared heating capacity divided by the power consumption under standard rated conditions. The seasonal coefficient of performance SCOP applies to a defined heating season. Class-specific values for individual outdoor temperature classes are called COPbin(Tj). From the SCOP, the regulation derives the seasonal space heating energy efficiency, which is decisive for minimum requirements and energy labels.

Physical limit: the Carnot value

A coefficient of performance above 1 does not contradict the law of conservation of energy. The heat pump does not simply convert electricity into heat. It uses it to pump heat from a lower to a higher temperature level. Thermodynamics sets an upper limit for this, the Carnot coefficient of performance: the absolute temperature of the heat sink in kelvin divided by the temperature lift. At a source temperature of 7 degrees and a flow temperature of 35 degrees, this gives roughly 308 K divided by 28 K, i.e. about 11. At minus 7 degrees and a flow temperature of 55 degrees, it is only about 328 K divided by 62 K, i.e. about 5.3. Because of losses in the compressor, heat exchangers and refrigerant circuit, real units reach only a fraction of this ideal value, often about half. This fraction is called the efficiency grade (Gütegrad). The calculation shows why a small temperature lift makes such a difference.

Coefficient of performance, efficiency and seasonal performance factor

The coefficient of performance is often confused with efficiency. For a boiler, an efficiency of 300 percent would be nonsensical if the energy in the fuel is taken as the reference. For a heat pump, by contrast, a COP of 3 or 4 is normal, because the heat delivered consists of electrical drive energy and harvested ambient heat. For contrast: an electric immersion heater converts almost all of the electricity into heat and thus has a coefficient of performance of approximately 1. A heat pump with a COP of 3 delivers about three times the heat from the same amount of electricity. Both heat electrically, but they burden power generation, the grid and the wallet very differently.

The COP must be distinguished from the seasonal performance factor (Jahresarbeitszahl). It relates the heat delivered over a year to the electricity used. This includes changing outdoor temperatures, domestic hot water preparation, defrosting, part load, cycling, auxiliary power for pumps and controls, and the behaviour of the occupants. Practical measured values are provided by a four-year field test by Fraunhofer ISE with 77 systems in one- to three-family houses in the existing building stock. The seasonal performance factors there ranged between 2.6 and 5.4. Air-to-water heat pumps averaged 3.4, ground-coupled systems 4.3. These values are clearly below typical data sheet figures for A7/W35. For energy and economic assessment, the measured seasonal performance factor is therefore usually more meaningful than a single COP.

Seasonal characteristic values such as the SCOP lie in between. They are not measured in the building but calculated from test points using standardised climate zones and load assumptions. This allows units to be compared. However, they do not replace consideration of the specific building, the heat emitters and the actual flow temperature. In the English-speaking world, the term Seasonal Performance Factor is often used for measured annual or seasonal values.

Why the temperature lift determines the coefficient of performance

The warmer the heat source and the lower the required flow temperature, the less work the compressor has to do. Ground and groundwater cool down less in winter than outdoor air and therefore often offer more favourable source temperatures. Underfloor heating or generously sized radiators manage with lower flow temperatures than small radiators that need very hot water. A suitably set heating curve and hydraulic balancing can reduce the required flow temperature and thus improve the real coefficient of performance.

The COP is therefore not an isolated quality feature of the unit. It describes the interplay of heat pump, heat source, building and heating system. A good unit can deliver mediocre values in a poorly set up environment. A solid system with large heat emitters, on the other hand, can run very efficiently. Incidentally, the Fraunhofer field test found no correlation between the building's year of construction and efficiency. The decisive factors were the temperatures in the heating circuit.

Significance for electricity consumption, the grid and costs

The COP determines how much electricity is needed for a given amount of heat. One kilowatt-hour of heat requires, arithmetically, 0.25 kilowatt-hours of electricity at a COP of 4, and 0.5 kilowatt-hours at a COP of 2. With millions of electrified heating systems, this difference changes electricity consumption, winter load and the need for expansion of the distribution grid.

For air-to-water heat pumps there is an additional unfavourable coincidence: low outdoor temperatures occur precisely when heating demand is high. In these hours the COP falls while the required heat output rises. The electrical power consumption therefore grows twice over. For grid operators, what matters above all is the power in critical hours, not the average annual consumption. Even a system with a good seasonal performance factor can contribute to peak load if it runs uncontrolled in grid situations that are already under strain. Below the bivalence point, an immersion heater with a coefficient of performance of about 1 may additionally switch on. In the Fraunhofer field test, however, immersion heaters supplied only a very small share of the work in the air-to-water systems.

The variable heating costs depend mainly on the electricity price and the real coefficient of performance. In addition there are investment, maintenance and, where applicable, grid charges. In simplified terms, the cost per kilowatt-hour of heat falls as the coefficient of performance rises. A thermal storage unit, the thermal mass of the building or time-variable tariffs can shift operation into cheaper hours. This has limits, however: if heating is done at a higher flow temperature for storage, or if the system has to catch up in very cold hours, the coefficient of performance falls. Part of the price advantage is then lost. Good control considers electricity price, grid signal, storage state and temperatures together.

Typical misinterpretations

  • A data sheet value as a general statement of efficiency: A COP at A7/W35 says little for a building that needs a 55 degree flow temperature in winter. Values at different test points cannot be compared directly.

  • "The heat pump has a COP of 4": This may refer to a laboratory value, an operating point, a monthly average, an SCOP or an expected seasonal performance factor. For planning, subsidies and grid planning, this makes a big difference.

  • COP as an efficiency above 100 percent: The coefficient of performance does not measure the conversion of electricity into heat, but how much heat is pumped per unit of drive power used.

  • COP as a climate balance: The climate impact additionally depends on how and when the electricity is generated. For time-resolved assessments, the marginal emission factor is relevant, for example. Low coefficients of performance in any case increase the demand for electricity, grids and flexibility.

  • Confusion with cooling characteristics: In cooling mode, data sheets usually state the energy efficiency ratio EER, which relates a different benefit to the input. It cannot be compared directly with the heating COP.

The coefficient of performance is a precise characteristic value for the efficiency of a heat pump process at a given operating point. It becomes meaningful only when source temperature, flow temperature, operating state, standard and system boundary are stated. Anyone using the term properly distinguishes between test value, seasonal standard value, measured seasonal performance factor and electrical load in critical hours.

Sources

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Edited by eCMS on 11.11.2026 18:02

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