Coefficient of Performance (COP)

What is the Coefficient of Performance?

The Coefficient of Performance, or COP, expresses the instantaneous ratio of heat delivered to electrical energy consumed by a heat pump at a specific test point. A COP of 3 means that for every kilowatt of electricity the pump draws, it outputs approximately 3 kilowatts of heat. COP is widely quoted by manufacturers and appears in product datasheets, but it is meaningless without the accompanying test-point temperatures. A headline claim of "COP 5" tells a Slovak buyer almost nothing about performance on a minus 10-degree January morning when radiators demand 50-degree water.

Why is COP not an efficiency rating?

Heat pump efficiency is often misunderstood. Unlike a boiler, which converts fuel into heat (bounded by the laws of thermodynamics at roughly 85-95 percent efficiency), a heat pump moves existing heat from one place to another. COP measures this movement: the ratio of heat energy shifted to electrical energy consumed. Because heat is already present in the outdoor air or ground (even at freezing temperatures, there is molecular motion), a COP greater than 1 does not violate any physical law. A COP of 4 does not mean the pump creates energy; it means it relocates four units of heat using one unit of electricity. The second and third law of thermodynamics set a theoretical ceiling called the Carnot limit, which defines the maximum possible COP for a given temperature difference. Real heat pumps achieve 40-60 percent of the Carnot limit due to compressor losses, heat-exchanger inefficiencies, and other irreversibilities.

What does the test-point notation mean?

European heat-pump standards (EN 14511, EN 14825) use a shorthand to specify operating conditions. A7/W35 means outdoor air at 7 degrees Celsius and heating-water outlet at 35 degrees Celsius. A minus 7/W35 (often written with a minus sign, sometimes as "minus 7 degrees/W35") means minus 7 degrees outdoor, same sink. These mild conditions are chosen for comparability across manufacturers, not because they represent typical Slovak winter use. On the coldest days in Central Europe, outdoor temperatures reach minus 15 degrees or lower, and radiators may demand 55-60 degree water to keep rooms warm. At these conditions, COP collapses.

How dramatically does temperature affect COP?

The relationship between COP and temperature lift is nearly exponential. Consider an air-to-water heat pump with a nameplate COP 4.5 at A7/W35 (28-degree lift: 7 outdoors, 35 water). As outdoor temperature drops and heating demand rises, COP deteriorates. The same pump at minus 10 outdoors with 50-degree water (60-degree lift) might achieve only COP 2. This is not a defect; it is physics. The solution is not a better pump but a lower heating-water temperature: buildings designed for radiant-floor heating or low-temperature heating systems require only 30-35 degree water, maintaining COP above 3 even in mid-winter. Every degree reduction in design flow temperature is worth more than buying a marginally better appliance.

This table illustrates why a ground-source heat pump drawing from stable ground temperatures can maintain higher COP than an air-source pump facing outdoor swings. It also shows why system design (minimizing water temperature) is often more cost-effective than choosing a marginally more efficient pump.

What real-world losses are not captured in COP?

Defrost cycles and electric auxiliary heating are excluded from standard test-point COP. When outdoor air temperature drops below roughly minus 5 degrees, frost accumulates on the evaporator coil of an air-source heat pump. The pump reverses cycle temporarily to thaw it, consuming electricity without delivering heat to the building. In severe cold, backup electric resistance heaters activate, dramatically lowering overall seasonal efficiency. Neither penalty is reflected in a nameplate COP figure. This is why season-averaged SCOP figures are essential for predicting annual costs: they incorporate these real-world parasitic loads.

What are EER and SEER for cooling?

The cooling equivalents of COP are EER (Energy Efficiency Ratio) and SEER (Seasonal Energy Efficiency Ratio). EER measures instantaneous cooling COP at a single test point (typically 35 degrees outdoors, 27 degrees indoors); SEER averages over a cooling season. A heat pump operating in reverse (cooling mode) has different COP values than heating mode, usually lower because it moves heat from indoors to a hotter outdoor environment. Some systems report separate heating COP and cooling COP; others report a combined seasonal figure. Never confuse COP with EER or SEER; they measure different directions and test conditions.

How should homeowners use COP information when comparing products?

COP is a marketing figure, not a buying metric. Compare SCOP values instead; they account for real seasonal performance. Verify that all products use the same standard (EN 14825) and the same climate zone. European manufacturers must declare SCOP for at least one continental, temperate, or Mediterranean climate zone; insist on the zone matching Slovakia's. Ensure the SCOP includes defrost and backup heating penalties. Finally, prioritize system design over component efficiency: lowering the heating-water temperature by 5 degrees yields more cost benefit than upgrading a pump with COP 4 to one with COP 5. A radiant-floor or low-temperature system paired with a modest pump often outperforms a high-COP pump feeding traditional radiators in annual savings.

What is the Carnot limit?

The maximum theoretical COP for any heat pump is set by the Carnot equation, which depends only on the absolute temperatures (in Kelvin) of the source and sink. For a pump lifting heat from 7 degrees Celsius outdoors to 35 degrees output, the Carnot COP is roughly 9. Real pumps achieve 40-60 percent of this limit: 4-5 COP. This ceiling explains why COP always drops as temperature lift grows. At minus 10 outdoors and 50-degree output, the Carnot limit is about 4; real pumps achieve 1.5-2. No engineering innovation can overcome this limit; it is fundamental to thermodynamics.