A heat pump moves heat instead of making it, so it can deliver more heat than the electricity it uses. The coefficient of performance (COP) measures that ratio, and it's the number that decides what a heat pump costs to run.
COP = heat delivered ÷ electricity used, in the same units. Electric resistance heat has a COP of 1; ENERGY STAR's cold-climate heat pumps must reach at least 1.75 at 5°F. A heat pump's HSPF2 rating converts to a seasonal COP by dividing by 3.412, so 7.8 HSPF2 is a seasonal COP of about 2.29.
The formula
COP = heat output ÷ electrical input. A heat pump using 1 kW of electricity to deliver 3 kW of heat has a COP of 3. ENERGY STAR describes heat pumps as delivering up to three times more heat energy than the electricity they consume.
For cooling, COP is the heat removed divided by the electricity used. EER and SEER express the same idea in BTU per watt-hour, which is why dividing by 3.412 converts them to COP.
How temperature changes COP
COP falls as the outdoor air gets colder, because the heat pump has to lift heat across a bigger temperature gap. Physics sets an upper limit, the Carnot limit: COP max = T hot ÷ (T hot − T cold), with temperatures in kelvin.
| Outdoor temperature | Maximum possible COP, delivering 95°F air |
|---|---|
| 47°F | 11.56 |
| 17°F | 7.11 |
| 5°F | 6.16 |
Real heat pumps reach well below these limits, but follow the same trend: the colder it is, the lower the COP. That's why ENERGY STAR tests cold-climate models at 5°F; see heat pumps in cold weather.
Instantaneous COP vs seasonal COP (SCOP)
A single COP figure describes one operating condition: a fixed outdoor temperature, indoor setpoint, and compressor speed. That is the instantaneous COP, the kind quoted at 47°F or 5°F on a spec sheet.
Seasonal COP, sometimes written SCOP, averages performance across a whole heating season, including mild and cold hours, defrost cycles, and any electric backup. It is always lower than the mild-weather instantaneous COP, because the cold hours and defrost pull the average down.
HSPF2 is a seasonal measure, so the 7.8 HSPF2 that converts to a seasonal COP of 2.29 is effectively an SCOP for a moderate climate. A spec sheet's 47°F COP of 3 to 4 and its seasonal COP of about 2.3 describe the same machine at different scopes.
COP and other ratings
| Rating | What it measures | Converting to COP |
|---|---|---|
| HSPF2 | Seasonal heating efficiency, BTU per watt-hour | ÷ 3.412: 7.8 HSPF2 is a seasonal COP of 2.29 |
| SEER2 | Seasonal cooling efficiency | ÷ 3.412: 15.2 SEER2 is 4.45 |
| EER2 | Cooling efficiency at one hot condition | ÷ 3.412: 11.0 EER2 is 3.22 |
Single-point COP figures in marketing, often 3 to 4, are measured in mild conditions. HSPF2 averages a whole heating season for a moderate climate, which is why it converts to a lower COP.
Reading COP off a spec sheet
A product spec sheet or AHRI Directory listing rarely prints COP directly; it prints SEER2, EER2, and HSPF2, which you convert by dividing by 3.412.
Take a listing showing 15.2 SEER2, 11.0 EER2, and 7.8 HSPF2. For cooling, 15.2 ÷ 3.412 is a seasonal cooling COP of 4.45, and 11.0 ÷ 3.412 is a COP of 3.22 at the hot rating point. For heating, 7.8 ÷ 3.412 is a seasonal heating COP of 2.29.
For cold-climate performance, read the low-temperature line instead of the seasonal one. ENERGY STAR's cold-climate criteria require a COP of at least 1.75 measured at 5°F, so a qualifying listing shows a 5°F COP at or above that figure.
Ground-source heat pumps and COP
A ground-source (geothermal) heat pump draws heat from the ground instead of outdoor air. The ground stays far milder than winter air, so the temperature gap the heat pump must bridge is smaller, which raises COP exactly when an air-source unit struggles most.
The physical ceiling shows the size of the advantage. Using the same Carnot limit, delivering 95°F air from 50°F ground gives 308 ÷ (308 − 283), about 12.3, against 6.16 from 5°F outdoor air. Real units reach well below these limits but keep the same ranking.
The site's comparison uses an assumed seasonal COP of 3.5 for the ground-source unit as an illustration, against about 2.4 for an 8.2 HSPF2 air-source model; see air-source vs. ground-source heat pumps. Check the rated efficiency of any system you are quoted, since ENERGY STAR certifies geothermal models separately from air-source ones.
What COP means for your bill
Cost of heat per million BTU = 293.07 kWh × your electricity rate ÷ COP. At 18.19 cents per kWh, the U.S. average for January to July 2026 (EIA):
| COP | Cost per million BTU of heat |
|---|---|
| 1.0 (resistance) | $53.31 |
| 1.75 | $30.46 |
| 2.5 | $21.32 |
| 3.0 | $17.77 |
| 4.0 | $13.33 |
A 95% gas furnace at an assumed $1.35 per therm delivers heat for about $14.21 per million BTU, so at average prices a heat pump needs a seasonal COP of about 3.75 to beat it; see gas vs. electric heating cost.
Frequently asked questions
What is a good COP for a heat pump?
A COP of 3 or more in mild weather is common; at 5°F, ENERGY STAR's cold-climate minimum is 1.75. Seasonal COP, from HSPF2, is usually 2.2 to 3.
How do I convert HSPF2 to COP?
Divide by 3.412. 8.1 HSPF2 is a seasonal COP of about 2.37.
Can COP be less than 1?
Not in normal heating operation: the compressor's own energy ends up as heat indoors, on top of the heat drawn from outside. Defrost cycles and the outdoor fan's power pull the effective figure down in deep cold.
Why is my heat pump's COP lower in winter?
It has to move heat across a larger temperature difference, which takes more electricity for each unit of heat.
What is the difference between COP and SCOP?
COP describes one operating condition, such as 47°F or 5°F, while seasonal COP (SCOP) averages a whole heating season, including mild hours, deep cold, and defrost. SCOP is always the lower number. HSPF2 ÷ 3.412 is effectively an SCOP for a moderate climate.
What is the COP of a geothermal heat pump?
A ground-source unit runs at a higher COP than an air-source one because the ground stays milder than winter air, shrinking the temperature gap it has to bridge. The site's worked comparison uses an assumed seasonal COP of 3.5 as an illustration, so check the rated efficiency of any system you are quoted. The Carnot ceiling drawing from 50°F ground and delivering 95°F air is about 12.3, far above what real units reach.
Sources & References
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