COP stands for Coefficient of Performance, and it’s the number that tells you how efficiently a heat pump converts electricity into heating or cooling. A heat pump with a COP of 3 delivers 3 units of heat for every 1 unit of electricity it consumes. The higher the COP, the less energy you’re paying for to stay comfortable.
How COP Is Calculated
The formula is simple: divide the heating or cooling energy your system produces by the electrical energy it uses.
COP = Heating (or Cooling) Output ÷ Electrical Energy Input
If a heat pump uses 1 kW of electricity and produces 4 kW of heat, its COP is 4. That might sound like it’s creating energy from nothing, but it isn’t. A heat pump doesn’t generate heat the way a furnace does by burning fuel. It moves heat from the outdoor air (or ground) into your home, and that process takes far less energy than creating heat from scratch. The electricity powers a compressor and fans that do the moving, while the heat itself comes from the environment.
An electric space heater or baseboard heater, by comparison, has a COP of exactly 1. It converts electricity directly into heat at a 1:1 ratio. A heat pump delivering a COP of 3 or 4 gives you three to four times as much warmth from the same amount of electricity. That’s the core reason heat pumps save money on energy bills.
What COP Numbers to Expect
Most modern air-source heat pumps achieve a COP between 3 and 5 under favorable conditions. At an outdoor temperature of 10°C (50°F), peak COP values around 5.3 to 5.5 are realistic for variable-speed units, based on lab testing by the National Renewable Energy Laboratory. That performance drops as the temperature outside falls, because the heat pump has to work harder to extract warmth from colder air.
The relationship is roughly linear. For every degree the outdoor temperature drops, the peak COP decreases by about 0.09. So at 0°C (32°F), you might see a peak COP around 4.3, and at -15°C (5°F), it could drop to roughly 2.9. Even at that lower number, the heat pump still produces nearly three times the heat of a standard electric heater using the same electricity.
Geothermal (ground-source) heat pumps generally have higher and more stable COP values, often between 3.5 and 5, because ground temperatures stay relatively constant year-round. They aren’t subject to the same swings that affect air-source systems in cold weather.
COP vs. SCOP: One Moment vs. a Whole Season
A single COP number is a snapshot. It tells you efficiency at one specific outdoor temperature under controlled test conditions. That’s useful for comparisons, but it doesn’t reflect what you’ll actually experience over an entire winter with fluctuating weather.
That’s where SCOP, or Seasonal Coefficient of Performance, comes in. SCOP averages the heat pump’s efficiency across a full heating season, accounting for mild days, cold snaps, and everything in between. European testing standards define three climate profiles for calculating SCOP: “average,” “warmer,” and “colder.” Each uses a different range of outdoor test temperatures, from 12°C down to -7°C for average climates, and as low as -15°C for colder ones. The indoor target is always 20°C (68°F).
If you’re comparing heat pumps, SCOP gives you a much better picture of real-world performance than a single COP figure. A unit with a high COP at 7°C but poor cold-weather performance could have a lower SCOP than a unit with slightly lower peak efficiency but better consistency across temperatures.
North American Efficiency Ratings
If you’re shopping for a heat pump in the U.S. or Canada, you’re more likely to see HSPF2 (Heating Seasonal Performance Factor 2) than SCOP. HSPF2 serves a similar purpose: it measures heating efficiency across a range of seasonal temperatures rather than at a single point. The key difference is the unit of measurement. COP is a simple ratio with no units, while HSPF2 is expressed in BTU per watt-hour. You can convert between them by dividing HSPF2 by 3.412.
For air-source heat pumps, HSPF2 and SEER2 (for cooling) are the standard efficiency ratings. For geothermal systems, COP and EER2 are used instead, because ground-source performance is measured at specific, stable temperatures rather than across a seasonal range.
Why Temperature Lift Matters
The single biggest factor affecting COP is the temperature difference between the heat source (outdoor air or ground) and the heat you want delivered inside. Engineers call this the “temperature lift.” The smaller the lift, the less work the compressor has to do, and the higher the COP.
This is why heat pumps paired with underfloor heating or low-temperature radiators perform better than those feeding traditional high-temperature radiators. Underfloor systems typically need water at 35 to 45°C, while older radiators might need 55 to 70°C. Reducing the delivery temperature by even 10 degrees noticeably improves efficiency.
It’s also why COP drops in extreme cold. At -15°C outside with a 20°C indoor target, the heat pump is bridging a 35-degree gap. At 7°C outside, it’s only bridging 13 degrees. The theoretical maximum COP (defined by the Carnot cycle from thermodynamics) is the ratio of the target temperature to that temperature difference. For a 35-degree lift with a 20°C target (293 K), the Carnot limit is about 8.4. Real-world systems achieve roughly 40 to 60% of that theoretical ceiling, depending on design and operating conditions.
How COP Affects Your Energy Bills
COP translates directly into money. If electricity costs you $0.15 per kWh and your heat pump runs at a COP of 3, every kWh of heat costs about $0.05. The same heat from an electric resistance heater costs the full $0.15. A natural gas furnace operating at 95% efficiency, with gas at $1.20 per therm, costs roughly $0.04 per kWh of heat, which is competitive, but that gap narrows or reverses depending on local energy prices.
When comparing heating options, the COP (or SCOP for seasonal estimates) is what lets you do this math. A higher COP doesn’t just mean “more efficient” in an abstract sense. It means fewer dollars per degree of warmth. If you’re choosing between two heat pump models and one has a SCOP of 4.2 while the other sits at 3.5, the first model will use about 17% less electricity over a heating season for the same comfort level. Over a decade of heating bills, that difference adds up.