f97.beERIK SCHMIDT’S WORKSHOP
← All terms

Glossary · Energy industry & energy transition

COP

COP Glossary Energy power system heat pump coefficient of performance Energy Efficiency

COP stands for Coefficient of Performance. For a heat pump in heating mode, it denotes the ratio between the heat output delivered and the electrical power consumed in a given operating state. A COP of 4 means: under the stated conditions, 1 kilowatt of electrical power yields 4 kilowatts of heat output. The missing 3 kilowatts do not come from the socket. They come from the environment, for example outdoor air, the ground, groundwater or waste heat. EU ecodesign law accordingly describes heat pump space heaters as appliances that use ambient heat from air, water or ground, or waste heat.

The COP has no unit. It is not a quantity of energy but a ratio of two power values at one point in time or at a defined test point. It must therefore be distinguished from power in kilowatts and from electricity consumption in kilowatt-hours. The COP shows how efficiently a heat pump converts electrical power into usable heat output in a particular situation. How much electricity it actually needs over a winter, by contrast, depends on many factors: the building's heat demand, outdoor temperatures, flow temperature, user behaviour, controls and operating time. For cooling mode there is a separate metric, the EER (Energy Efficiency Ratio).

In German, the COP is usually called Leistungszahl. The translation fits as long as it is clear that this is not a fixed device value. A heat pump does not have "the" COP in the way an engine has a fixed rated output. The COP depends on how large the temperature difference is that the heat pump has to bridge. The warmer the heat source and the lower the required heating temperature, the higher the COP. An air-to-water heat pump works markedly more efficiently with mild outdoor air and underfloor heating than in frost with high flow temperatures for old radiators. Note the abbreviation: in climate policy, COP also stands for the "Conference of the Parties", that is, the UN climate conferences. That has nothing to do with the coefficient of performance.

Physical upper limit: the Carnot COP

The maximum possible COP follows from thermodynamics. The ideal Carnot COP for heating is the absolute temperature of the heat sink divided by the temperature difference between sink and source, both in kelvin. Worked example with a 7 °C source (about 280 K) and a 35 °C sink (about 308 K): 308 divided by 28 gives roughly 11. With a −7 °C source and a 55 °C sink, only about 328 divided by 62 remains, i.e. around 5.3. Real devices achieve only a fraction of this ideal value because of losses in the refrigerant circuit, often around half. The example shows the core of the matter: the temperature lift determines efficiency. Values above this ideal value are physically impossible.

Distinction from efficiency, JAZ and SCOP

The COP is often confused with an efficiency. An efficiency relates the usable energy to the energy input of a conversion. For an immersion heater it is close to 1: one kilowatt-hour of electricity becomes almost exactly one kilowatt-hour of heat. A heat pump achieves a COP above 1 because it lifts heat from the environment to a higher temperature level. For the heat pump, electricity is drive energy for a thermodynamic process, not the sole heat source.

This does not violate the conservation of energy. The COP only compares the electrical drive power with the heat output delivered. Environmental heat is not counted as energy input here, but physically it belongs to the heat balance. Advertising slogans such as "400 percent efficiency" sound catchy but are technically misleading. More accurate is: under certain conditions, the heat pump delivers four times as much heat output as it takes in as electrical power.

The seasonal performance factor (Jahresarbeitszahl, JAZ) must be distinguished from the COP. It relates the amount of heat delivered over a year to the amount of electricity consumed in the same year, ideally both measured in operation. Because it is based on energy quantities, it says more about operating costs and annual electricity consumption. The SCOP (Seasonal Coefficient of Performance), by contrast, is a standardised calculated value. Under EU Ecodesign Regulation (EU) No 813/2013, it is the quotient of a reference annual heating demand and the annual energy consumption calculated for it. The basis is standardised climate and part-load conditions described in the standard EN 14825. From the SCOP, the seasonal space heating energy efficiency ηs is calculated in percent. To do so, the SCOP is divided by a conversion coefficient of 2.5, which converts electricity generation into primary energy, and then corrected. The SCOP makes it easier to compare devices but does not replace an assessment of the specific building. Related is the term Seasonal Performance Factor.

A COP from a data sheet always applies only to defined test conditions. In Europe, these test points are measured according to the test standard EN 14511. For air-to-water heat pumps, A7/W35 means: outdoor air 7 °C, heating water flow 35 °C. Other common points are A2/W35, A−7/W35 or variants with a higher flow temperature such as W45 or W55. For brine-to-water devices, B0/W35, for example, stands for a brine temperature of 0 °C. A−7/W55 describes a markedly less favourable situation: frosty outdoor air and a high flow temperature. Two devices can look similarly good at A7/W35 and differ markedly at low temperatures or high flow temperatures. Anyone who compares only the highest COP in the brochure often compares a favourable test point rather than behaviour in critical heating operation. COP values can only be compared reliably at the same test point.

Why the COP matters in the electricity system

The COP links building heat with electricity demand. When heat pumps replace many oil and gas heating systems, the demand for fossil final energy generally falls while electricity consumption rises. However, this additional electricity demand does not correspond to the quantity of fuel previously burned. A heat pump with a seasonal performance factor of 3 needs only about one third of the electricity that direct electric heating would use for the same amount of heat. Compared with oil or gas heating, the energy chain shifts: less fuel in the building, more electricity demand on the grid, and thus a greater role for electricity generation and grid capacity.

For the operation of the electricity system, it is not only the annual quantity of electricity that counts. Heat pumps run mainly in cold periods, that is, precisely when heat demand is high and air-based systems work less efficiently. A simplified example: at an outdoor temperature of 5 °C, a house needs 3.5 kilowatts of heating output. At a COP of 4.5, the heat pump then draws about 0.8 kilowatts of electricity. At −10 °C, demand rises to 8 kilowatts and the COP drops to about 2.5. That makes about 3.2 kilowatts, i.e. roughly four times as much. This effect acts on peak load, local grid loading and the design of the distribution grid. Even a system with a good seasonal performance factor can cause a noticeable grid load in individual hours if many systems run simultaneously in cold weather.

The COP thus makes visible a technical dependency that is easily lost in simplified debates about electrification: efficiency depends on the temperature level. Buildings with low heat demand, large heating surfaces, low flow temperature and good controls relieve the electricity system more than poorly renovated buildings with high system temperatures. A poor starting position does not automatically rule out a heat pump. But it changes electricity demand, operating costs and requirements for the grid.

Technical conditions behind the metric

A heat pump works with a refrigeration cycle. A refrigerant evaporates at low temperature and absorbs heat in the process. It is then compressed, releases heat to the heating system at a higher temperature and is subsequently expanded again. The compressor needs electrical energy. The greater the temperature difference between heat source and heating system, the more work it has to do, and the lower the COP.

With air-to-water heat pumps, the outdoor air is cold in winter and the evaporator can ice up. Every defrost cycle costs energy and reduces usable efficiency, especially at temperatures around freezing point and high humidity. Brine-to-water heat pumps with borehole heat exchangers or ground collectors usually have more stable source temperatures, but cost more to develop and raise different permitting questions. Water-to-water heat pumps can work very efficiently. However, they require sufficient groundwater, a permit under water law and suitable water quality.

The question of what is measured as well (the system boundary) also influences the stated COP. Test standards determine which shares of auxiliary energy for pumps, fans and controls are included in the laboratory value. External circulation pumps, buffer tanks or the electric backup heater are generally not, or not fully, included. For the electricity bill and grid load, however, these consumers count. In practice, therefore, what matters is less the device COP alone than the efficiency of the whole system: heat generator, storage, hydraulics, heating surfaces, controls and building envelope.

Typical misinterpretations

A common misconception is that a high COP automatically makes electricity costs or grid loading unproblematic. The COP does reduce electricity demand per unit of heat. But it does not cancel out the fact that heat pumps run mainly in cold weather. If many systems run at the same time and little load can be shifted, even efficient heat generation may require grid reinforcement or grid-supportive control.

A second oversimplification is to assess heat pumps solely by their COP under standard conditions. For owners, grid operators and energy policy, operation in unfavourable hours is often more important than the best value in the brochure. Costs depend not only on kilowatt-hours per year but also on connection capacity, grid expansion, tariff structure and controllability. In Germany, § 14a of the Energy Industry Act (Energiewirtschaftsgesetz) regulates the grid-oriented control of controllable consumption devices. The Federal Network Agency (Bundesnetzagentur) specifies it by determination. Since 1 January 2024, the following applies in principle: newly commissioned heat pumps on the low-voltage grid with more than 4.2 kilowatts of electrical connection capacity are controllable. What matters is the electrical power, not the heat output. The grid operator may temporarily curtail electricity draw in case of impending overload, but in principle not below a minimum power of 4.2 kilowatts. Higher values apply to larger systems. In return, grid charges are reduced. Transitional rules apply to older systems with earlier blocking-period agreements. Dynamic electricity tariffs, thermal storage and the thermal mass of the building can additionally change the effect on the grid if they are integrated in a technically sensible way.

The COP also says nothing directly about climate impact. A heat pump with a high COP needs less electricity for the same amount of heat. However, emissions additionally depend on how the electricity is generated, which period is considered and which heating system is replaced. The more electricity comes from renewable sources, the better the climate balance of electric heat generation. The COP remains the key variable for how much electricity is needed for the heat provided.

The COP is particularly important when comparing with direct electric heat. Immersion heaters, night storage heaters or electric boilers convert electricity almost one to one into heat, so their COP is practically 1. A heat pump uses the same high-grade energy carrier, electricity, but multiplies the usable heat through environmental heat. Many heat pumps have an electric backup heater for peak loads or emergency operation. If it has to run permanently, for example because the system is undersized or the flow temperature is too high, the real performance factor deteriorates markedly. Whether a system electrifies efficiently or essentially runs as direct electric heating is then decided not by the label but by actual operation.

The COP is thus a precise metric for a particular operating state, but not a complete verdict on a heating system. It explains why heat pumps can use electrical energy very effectively for heat. But it also shows why building condition, temperature level, load profile, grid connection and market rules belong in the assessment. Its strength is the clean link between thermodynamics and electricity demand. Its limit lies where an instantaneous value is used to derive a statement about a whole year, a building or the electricity system.

Sources

Related glossary entries

Edited by eCMS on 11.11.2026 18:01

© 2026 Erik Schmidt. All rights reserved. Quotations with a link to the source are welcome.