COP stands for Coefficient of Performance, and it’s the primary way to measure how efficiently a heating or cooling system converts energy into useful work. It’s a simple ratio: the amount of heating or cooling a system delivers divided by the electrical energy it consumes. A COP of 3.0 means the system produces three units of heating or cooling for every one unit of electricity it uses.
How COP Is Calculated
The formula is straightforward: COP equals the useful heating or cooling output divided by the electrical power input. Both values are measured in watts, which makes COP a dimensionless number with no units attached. This is one reason it’s useful for comparing equipment across different countries and measurement systems.
A standard electric space heater converts electricity to heat at a 1:1 ratio. Every watt of electricity becomes a watt of heat, giving it a COP of exactly 1.0. That’s your baseline. Heat pumps beat this baseline because they don’t generate heat directly. Instead, they move heat from one place to another, which takes far less energy than creating heat from scratch. That’s why a heat pump can have a COP of 3 or 4, delivering three or four times more heating energy than the electricity it consumes.
Typical COP Values by System Type
COP varies significantly depending on the type of equipment and the conditions it operates under. Here’s what you can generally expect:
- Air source heat pumps typically fall in the range of 2 to 6, depending on outdoor temperature and operating conditions.
- Ground source (geothermal) heat pumps tend to run higher because they draw heat from underground, where temperatures are more stable. A study of a ground source system in Southern Germany measured a heating COP of 3.9 on a typical winter day and a cooling efficiency ratio of 8.0 on a typical summer day.
- Air-cooled commercial chillers generally operate around 2.8 to 3.1 COP.
- Water-cooled commercial chillers are more efficient, ranging from about 4.2 COP for smaller reciprocating units up to 5.5 COP for large centrifugal chillers.
Higher COP means lower operating costs. A system with a COP of 4.0 uses half the electricity of a system with a COP of 2.0 to deliver the same amount of heating or cooling.
Why COP Changes With Temperature
COP is not a fixed number. It shifts constantly based on the temperature difference between the indoor space and the outdoor environment. The smaller that gap, the less work the system has to do, and the higher the COP climbs.
Research published in Heliyon quantified this relationship clearly. Compared to a baseline outdoor temperature of 7°C (about 45°F), raising the ambient temperature to 20°C (68°F) boosted COP by up to 35%. Dropping the outdoor temperature to -10°C (14°F) reduced COP by 26%. This is why heat pumps perform best in mild climates and lose efficiency during extreme cold. It’s also why ground source heat pumps outperform air source models: the ground stays warmer than the air in winter, giving the system a smaller temperature gap to work against.
There’s a theoretical ceiling for COP set by the laws of thermodynamics, known as the Carnot limit. No real system can reach it because it would require infinitely slow operation, producing zero actual output. But it provides a useful benchmark. Real-world systems typically achieve a fraction of their Carnot limit due to mechanical losses, friction, and the practical constraints of moving refrigerant through coils and compressors.
COP vs. EER vs. SEER
If you’ve shopped for air conditioning, you’ve probably seen EER and SEER ratings alongside or instead of COP. These all measure efficiency, but they do it differently.
COP is an instantaneous snapshot measured in watts in and watts out. It tells you how efficient the system is at a single operating point. EER (Energy Efficiency Ratio) measures cooling output in BTUs against electrical input in watt-hours, so it captures performance over time at a specific temperature difference. SEER (Seasonal Energy Efficiency Ratio) goes further by averaging efficiency across an entire cooling season, accounting for the range of outdoor temperatures the system will face. Because milder days boost efficiency, SEER is always higher than EER, typically by 15% to 35%.
You can convert between them. To get COP from an EER value, multiply EER by 3.412 (or more precisely, 3600 divided by 1055). A SEER of 13, for example, translates to roughly an EER of 11.2, which works out to a COP of about 3.6. If you’re looking at commercial chiller specs listed in kW per ton, divide 3.517 by that number to get COP.
What COP Means for Energy Costs
COP translates directly to your electricity bill. If you’re heating a space that needs 10 kW of heat, a system with a COP of 1.0 (like an electric resistance heater) draws 10 kW of electricity. A heat pump with a COP of 4.0 delivers the same 10 kW of heat while drawing only 2.5 kW of electricity. At $0.15 per kWh, that’s the difference between $1.50 and $0.375 per hour of operation.
When comparing equipment, look at COP values measured under conditions similar to your climate. A heat pump advertised with a COP of 5 was likely tested under ideal conditions. In a cold northern winter, you might see that drop to 2.5 or lower. Seasonal COP (sometimes called SCOP) gives a more realistic picture of year-round performance. Ground source systems tend to maintain more consistent COP throughout the year, though their higher installation cost means the payback period is longer.
For commercial buildings, even small COP improvements add up fast. Moving from a chiller plant running at 4.2 COP to one running at 5.5 COP reduces compressor electricity consumption by roughly 24% for the same cooling load. At the scale of a large commercial building, that can represent tens of thousands of dollars annually.