COP stands for coefficient of performance, and it’s the simplest way to measure how efficiently a heat pump heats your home. It’s a ratio: the amount of heat energy delivered divided by the electrical energy used to run the system. A heat pump with a COP of 3 produces three units of heat for every one unit of electricity it consumes, making it three times more efficient than a basic electric heater.
How COP Is Calculated
The formula is straightforward: COP equals the heat output divided by the electrical energy input. If a heat pump delivers 9,000 BTUs of heat while consuming 3,000 BTUs worth of electricity, its COP is 3.0. A higher number always means better efficiency. Unlike a furnace or space heater, which can never exceed a COP of 1.0 (since they convert electricity or fuel directly into heat), heat pumps move existing heat from outdoor air or the ground into your home. That’s why they routinely achieve COPs of 3 to 6, effectively delivering several times more energy than they consume.
There is a theoretical ceiling set by thermodynamics, known as the Carnot limit. This maximum COP depends entirely on the temperature difference between your heat source (outdoor air, ground) and your desired indoor temperature. The smaller that gap, the higher the theoretical limit. In practice, no real heat pump reaches the Carnot limit because of mechanical losses, but it explains why heat pumps perform best in mild weather and why ground-source systems, which pull heat from a more stable temperature, tend to outperform air-source models.
How Outdoor Temperature Affects COP
COP is not a fixed number. It changes constantly based on conditions, and the biggest variable is outdoor temperature. As it gets colder outside, your heat pump has to work harder to extract heat from the air, and COP drops. A unit that runs at a COP of 4.3 on a mild day near 50°F might fall to around 2.0 or lower when temperatures dip well below zero.
Research testing four ductless mini-split heat pumps across a range of outdoor temperatures found that peak COP follows a roughly linear relationship with temperature. For every degree Celsius the outdoor temperature drops, peak COP falls by about 0.09. At 10°C (50°F), the best-performing units achieved COPs above 5. At -25°C (-13°F), COP dropped significantly but still remained above 1.0, meaning the heat pump was still delivering more heat energy than the electricity it consumed.
This is why manufacturers and certification programs test heat pumps at specific low temperatures. To earn the Energy Star cold climate designation, a heat pump must achieve a COP of at least 1.75 at 5°F (-15°C). That threshold ensures the unit still meaningfully outperforms straight electric resistance heating even in harsh winter conditions.
COP vs. HSPF2 and SEER2
COP measures efficiency at a single moment under specific conditions (a particular outdoor temperature and heating load). That makes it useful for comparing performance at a given operating point, but it doesn’t tell you how the system performs across an entire heating or cooling season. That’s where seasonal ratings come in.
HSPF2 (Heating Seasonal Performance Factor 2) captures total heating output over an entire season divided by total electricity consumed. It accounts for the fact that outdoor temperatures vary day to day and that the unit cycles on and off. Think of COP as a snapshot and HSPF2 as the full movie. SEER2 (Seasonal Energy Efficiency Ratio 2) does the same thing but for cooling mode. Both use BTUs and watt-hours rather than the unitless ratio of COP, so the numbers look different, but they’re measuring the same basic concept: how much useful heating or cooling you get per unit of energy spent.
You can roughly convert HSPF2 to an average seasonal COP by dividing by 3.412 (the conversion factor between BTUs and watt-hours). An HSPF2 of 8.0, which is the Energy Star Most Efficient threshold for ducted split systems in 2025, translates to a seasonal average COP of about 2.3. That average is lower than the peak COP you’d see on a mild day because it includes the coldest hours of winter when the system works hardest.
Air-Source vs. Ground-Source COP
Air-source heat pumps pull heat from outdoor air and are the most common type installed in homes. Their COP typically ranges from about 2.5 to 4.5 under normal operating conditions, with performance dropping as temperatures fall. Modern cold-climate models have pushed the lower end of that range higher, maintaining useful efficiency even at subzero temperatures.
Ground-source (geothermal) heat pumps exchange heat with the earth, where temperatures stay relatively stable year-round, usually between 45°F and 60°F depending on your region. Because the temperature gap between the source and your home stays small and consistent, ground-source systems maintain higher and more stable COPs, often in the range of 3.5 to 5.5 through the heating season. The tradeoff is a significantly higher installation cost due to the underground loop system required.
What COP Means for Your Energy Bills
COP translates directly into operating cost. If electricity costs you $0.15 per kilowatt-hour and your heat pump runs at an average COP of 3, you’re effectively paying $0.05 per kilowatt-hour of heat delivered. A natural gas furnace operating at 95% efficiency, with gas priced at $1.20 per therm, costs roughly $0.04 per kilowatt-hour of heat. At a COP of 3, the heat pump is close to cost parity with gas. At a COP of 4 or higher, it’s cheaper.
This is why COP matters so much when choosing a heat pump: even small differences in COP compound over thousands of hours of operation each winter. A system that averages a COP of 3.5 instead of 2.5 uses about 30% less electricity for the same amount of heating. If you live in a climate where winter temperatures regularly drop below 20°F, pay close attention to COP ratings at low temperatures rather than peak values measured in mild conditions. The cold-weather COP is what will determine your actual energy bills during the months when heating demand is highest.