COP, or coefficient of performance, is a number that tells you how effectively a heating or cooling system moves energy compared to the energy it consumes. Unlike traditional efficiency, which caps out at 100%, COP regularly exceeds 1.0 because heat pumps and refrigerators don’t create heat from scratch. They move it from one place to another, and moving heat takes less energy than generating it. A home heat pump with a COP of 3, for example, delivers three units of heat for every one unit of electricity it uses.
How COP Is Calculated
The formula changes depending on whether the system is heating or cooling, because the “useful output” is different in each case.
For a heat pump warming a building, COP equals the heat delivered to the warm space divided by the work (electricity) put in:
COP (heating) = Qh / W
For a refrigerator or air conditioner, the goal is removing heat from a cold space, so COP equals the heat pulled out of that cold space divided by the work input:
COP (cooling) = Qc / W
In both cases, W is the electrical energy consumed by the compressor. The relationship tying everything together is straightforward: the heat rejected to the warm side (Qh) equals the heat absorbed from the cold side (Qc) plus the work input (W). This is why a heat pump’s COP in heating mode is always at least 1.0. Even if the system were terrible at moving heat, the electrical energy itself would still end up as heat in the warm space.
Why COP Can Be Greater Than 1
Traditional thermal efficiency measures how well an engine converts heat into work, and the laws of thermodynamics guarantee that ratio stays below 100%. COP flips that relationship. Because a heat pump’s COP is the inverse of a heat engine’s efficiency, and because no heat engine can be 100% efficient, COP for heating is always greater than 1. This isn’t free energy. The system is harvesting low-grade heat from outdoor air, the ground, or water and concentrating it indoors. The electricity powers the compressor that makes this transfer possible, but most of the delivered energy comes from the environment, not from the electrical grid.
A cooling system’s COP can dip below 1 under extreme conditions, but a well-designed residential unit typically operates well above that threshold.
The Carnot Limit
No real system can beat the theoretical maximum set by the Carnot cycle, which represents a perfectly reversible process with zero friction and no heat losses. The Carnot COP depends entirely on the temperatures of the hot and cold sides, measured in absolute units (Kelvin):
Carnot COP (cooling) = TL / (TH − TL)
Carnot COP (heating) = TH / (TH − TL)
The key insight here is the denominator: the temperature difference between the hot and cold sides. The smaller that gap, the higher the theoretical COP. A refrigerator keeping food at 4°C in a 22°C kitchen has a much smaller temperature gap than a freezer holding −18°C, which is why refrigerators are inherently more efficient than freezers. Real-world systems reach roughly 30% to 50% of their Carnot limit due to friction, pressure drops, and heat exchange inefficiencies.
How Temperature Affects Real-World COP
The Carnot formula predicts it, and real-world data confirms it: COP drops as the temperature difference between source and destination grows. For air-source heat pumps, this means performance swings with the weather. At ambient temperatures above 7°C (about 45°F), COP improves noticeably. One study in Heliyon found that raising the outdoor temperature from 7°C to 20°C boosted COP by roughly 35% in heating mode. Going the other direction, dropping to −10°C (14°F) cut COP by about 26%.
The reason is mechanical. Colder outdoor air forces the compressor to work harder to bridge a larger pressure gap between the evaporator (cold side) and condenser (warm side). That extra compressor work increases W in the denominator without a proportional increase in heat output, dragging COP down. This is also why setting your thermostat higher in winter costs disproportionately more energy: you’re widening the temperature lift the system has to overcome.
Typical COP Values by System Type
Different heating and cooling technologies cluster around different COP ranges, which is useful for comparing options:
- Electric resistance heater: COP of exactly 1.0. All electricity converts to heat, but no environmental heat is harvested.
- Air-source heat pump (heating mode): Typically 2.5 to 4.0 under moderate conditions. ENERGY STAR’s “Most Efficient 2025” criteria require a COP of at least 1.75 even at 5°F (−15°C), reflecting how much performance drops in extreme cold.
- Ground-source heat pump: Generally more efficient across a full year than air-source systems because underground temperatures stay relatively stable (around 10–15°C in most climates). While an air-source unit may outperform on mild days when outdoor air is warmer than the ground, a ground-source system holds its advantage during the coldest weather, exactly when you need heat most.
- Household refrigerator: COP in the range of 1.5 to 3.0, depending on design and the temperature difference between the interior and the kitchen.
- Window air conditioner: COP of roughly 2.5 to 3.5 under standard rating conditions.
COP vs. SEER and HSPF
If you’ve shopped for HVAC equipment, you’ve probably seen ratings like SEER (for cooling) and HSPF (for heating). These are seasonal performance metrics that average COP across a range of outdoor temperatures over a typical year. COP itself is a snapshot at one specific operating condition. SEER and HSPF give a fuller picture of what you’ll actually experience over a season, but they’re built from COP measurements at multiple test points.
To roughly convert SEER to COP, divide the SEER value by 3.41 (since SEER uses BTU/h per watt while COP is a pure ratio). A 20-SEER air conditioner, for example, has an average seasonal COP of about 5.9 for cooling. For HSPF, the same conversion applies: divide by 3.41.
Why COP Matters for Energy Costs
COP translates directly into your energy bill. If your heat pump has a COP of 3, you’re paying for one kilowatt-hour of electricity but getting three kilowatt-hours’ worth of heating. Compare that to a gas furnace at 95% efficiency: for every unit of gas energy, you get 0.95 units of heat. Even when electricity costs more per unit than natural gas, a heat pump with a COP of 3 can be cheaper to run because it delivers so much more heat per unit of energy purchased.
This comparison breaks down in very cold climates where COP plummets. If outdoor temperatures regularly sit below −10°C and your air-source heat pump’s COP drops toward 1.5 or lower, the cost advantage over gas narrows or disappears. That’s precisely why ground-source systems, with their stable source temperatures, remain attractive in cold regions despite higher installation costs.