The coefficient of performance, or COP, is a ratio that tells you how efficiently a heating or cooling system uses energy. It compares the amount of useful heating or cooling a system delivers to the amount of electricity it consumes. A COP of 3, for example, means the system produces three units of heating or cooling energy for every one unit of electrical energy it uses. Unlike percentage-based efficiency ratings that cap out at 100%, COP values routinely exceed 1 because heat pumps and refrigerators move thermal energy rather than creating it from scratch.
How COP Is Calculated
The basic formula is straightforward: divide the useful energy output (heating or cooling) by the energy input (work or electricity consumed). For a cooling system like a refrigerator or air conditioner, that looks like:
COP = Q / W
Where Q is the amount of heat removed from the cooled space and W is the electrical energy consumed. For a heat pump in heating mode, Q represents the heat delivered to the warm space instead. The distinction matters because a heat pump delivering warmth always has a higher COP than the same system providing cooling, since the heating side captures both the moved heat and the energy used to move it.
What COP Values Look Like in Practice
Real-world COP values vary widely depending on the type of system and operating conditions. Air-source heat pumps typically achieve a COP between 2.0 and 5.4 at around 8°C (46°F) outdoor temperature. That means a unit with a COP of 5 transfers 5 kWh of heat for every 1 kWh of electricity it consumes. As outdoor temperatures drop, so does the COP. At -8°C (about 18°F), the same types of heat pumps deliver COP values ranging from 1.1 to 3.7.
Ground-source (geothermal) heat pumps perform more consistently because underground temperatures stay relatively stable year-round. Their COP typically ranges from 3.5 to 5.0, which generally surpasses air-source systems, especially in cold climates where air temperatures swing dramatically.
Refrigeration systems operate at lower COP values because they maintain much colder temperatures. Commercial cold storage units running at -25°C achieve COP values around 1.60 to 1.71. Industrial quick freezers holding -35°C are even lower, with values between 1.20 and 1.32. The colder the target temperature, the harder the system has to work, and the lower the COP drops.
Why Temperature Difference Matters
The single biggest factor affecting COP is the temperature difference, or “temperature lift,” between the cold side and the warm side of the system. A heat pump pulling warmth from 10°C outdoor air and delivering it to a 20°C room only has to bridge a 10-degree gap. The same heat pump trying to extract heat from -15°C air and push it to 20°C faces a 35-degree gap, and its COP drops sharply.
This relationship is baked into the physics. The theoretical maximum COP for any cooling system is set by the Carnot limit: COP = Tc / (Th – Tc), where Tc is the cold-side temperature and Th is the warm-side temperature (both measured on an absolute scale like Kelvin). For a heat pump in heating mode, the Carnot limit is: COP = Th / (Th – Tc). No real system can reach these theoretical maximums due to friction, imperfect heat exchange, and other losses, but the formulas show why smaller temperature differences always mean better efficiency. Every additional degree of lift costs you performance.
COP vs. EER and SEER
If you’ve shopped for an air conditioner or heat pump, you’ve probably seen EER (Energy Efficiency Ratio) and SEER (Seasonal Energy Efficiency Ratio) on the spec sheet. These are related to COP but use different units and testing conditions.
- EER measures cooling output in BTU per hour divided by watts of electricity. You can convert it to COP by dividing EER by 3.412. So an air conditioner with an EER of 12 has a COP of about 3.5.
- SEER represents seasonal average performance rather than a single operating point. It accounts for varying temperatures over an entire cooling season. For single-speed units with a SEER of 16 or below, you can estimate EER using the formula: EER = -0.02 × SEER² + 1.12 × SEER. From there, divide by 3.412 to get COP.
COP gives you a snapshot of efficiency at one specific set of conditions. SEER gives you a broader seasonal picture. Neither is “better” as a metric; they just answer different questions. When comparing systems across categories (heat pumps, chillers, refrigerators), COP is the universal language.
What a Higher COP Means for Energy Costs
COP translates directly to your energy bill. If you have an electric resistance heater, it converts electricity to heat at a 1:1 ratio, giving it a COP of exactly 1. A heat pump with a COP of 3 delivers the same amount of heat using one-third the electricity. Over a heating season, that difference compounds into significant savings.
The practical takeaway: when comparing systems, a higher COP at your local climate’s typical temperatures matters more than a high COP measured under ideal lab conditions. A ground-source system holding a steady COP of 4.0 through winter will cost less to run than an air-source unit rated at COP 5.0 under mild conditions but dropping to 1.5 when temperatures plunge. Matching the system to your actual operating conditions is what determines real-world savings.