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Cop Explained
Quick answer: Coefficient of performance (COP) is heat delivered divided by electricity used, both in the same units. A COP of 3 means three units of heat for every unit of electricity. COP changes with outdoor temperature, falling as it gets colder, so seasonal ratings such as HSPF2 give a better picture of yearly efficiency.
The basic idea
Electric resistance heaters turn each kilowatt-hour of electricity into one kilowatt-hour of heat, a COP of 1. A heat pump uses electricity to move heat from outdoors, so it can deliver more heat than the electricity it uses. The extra comes from the outdoor air. The smaller the temperature lift between outdoor air and the indoor supply, the higher the COP.
Why COP falls in the cold
On a 47°F day the compressor lifts heat a modest amount and COP may be 3 to 4.5. At 5°F it lifts much further and COP may drop to around 1.5 to 2.5 depending on the model. Defrost cycles also use energy. That is why the same heat pump costs more per hour to run in January than in October.
COP and HSPF2
HSPF2 is a seasonal average for a defined test climate, measured in BTU of heat per watt-hour of electricity. Dividing HSPF2 by 3.412 gives an approximate average seasonal COP. An HSPF2 of 7.5 is about 2.2; 9 is about 2.6. Your real seasonal COP will be lower in colder climates than the test region and higher in milder ones. See HSPF and SEER explained.
Using COP to compare with other fuels
To compare cost per unit of heat, divide the electricity price by COP. At $0.15/kWh and COP 2.5, heat costs $0.06 per kWh of heat. A gas furnace at 95 percent efficiency with gas at $1.30/therm delivers heat at about $1.30 ÷ (29.3 × 0.95) ≈ $0.047 per kWh of heat. Here gas is cheaper; at COP 3.5 the heat pump would be about $0.043 and cheaper. The operating cost calculator does this for a whole season.
Break-even COP
The break-even COP is the COP at which the heat pump and another fuel cost the same. Divide the electricity price by the other fuel's cost per kWh of heat. In the example above it is 0.15 ÷ 0.047 ≈ 3.2. Above that COP, the heat pump is cheaper; below it, gas is. Some dual-fuel systems use this logic to switch to the furnace below a set outdoor temperature.
COP at a glance
| Outdoor temp | Typical standard unit | Typical cold-climate unit |
|---|---|---|
| 47°F | 3.0–4.0 | 3.5–4.5 |
| 17°F | 2.0–2.6 | 2.4–3.0 |
| 5°F | 1.5–2.0 | 1.8–2.5 |
These ranges are general; read your model's data.
Worked example
Figures below are illustrative examples (2026-10-29), not quotes or guarantees.
A house needs 20,000 BTU/h of heat on a 17°F night. At COP 2.5 the heat pump draws 20,000 ÷ 3,412 ÷ 2.5 ≈ 2.3 kW. Over 10 hours that is 23 kWh, about $3.45 at $0.15/kWh. Resistance heat (COP 1) would use about 59 kWh, roughly $8.80.
What lowers real-world COP
- Dirty filters or coils restricting airflow.
- Refrigerant charge that is off, which needs a licensed technician to check.
- Large thermostat setbacks that bring on backup heat.
- Ducts leaking into unheated spaces.
- Snow or ice blocking the outdoor coil.
COP versus HSPF2
HSPF2 is a seasonal rating that blends performance across a modeled heating season, while COP is a snapshot at one outdoor temperature and load. Two units with similar HSPF2 can have different COP at 5°F, which matters most in cold regions. When comparing models, check both: HSPF2 for the general season and the low-temperature COP for your coldest weeks.
Why COP changes with load
Variable-speed compressors are often most efficient at partial speed, so COP can be higher on a mild day when the unit runs slowly than at full output. Oversized units cycle more and spend less time in that efficient range. This is one reason right-sizing matters: a correctly sized variable-speed unit spends more hours near its best COP.
Estimating cost with COP
To estimate the electricity cost of a unit of heat, divide your electricity price by COP. If electricity costs a set amount per kWh and COP is 3, each kWh of heat costs one third of that price. Compare that to the price of the same heat from gas, oil or propane after accounting for furnace efficiency. Our operating cost calculator does this arithmetic with your local prices; results are planning estimates only.
Factors that lower real-world COP
Low refrigerant charge, dirty coils, restricted ductwork, frequent defrost cycles and supply air leaking into unconditioned attics all reduce delivered COP. Ducts are a frequent culprit in older homes: a heat pump can perform as rated while a third of its output leaks into a crawlspace. Sealing and insulating ducts is often one of the best value improvements.
Measuring COP at home
Homeowners rarely measure COP directly, but a smart thermostat's runtime report combined with an energy monitor on the heat pump circuit gives a rough picture. Compare kWh used on a few days with similar outdoor temperatures before and after maintenance. A noticeable rise in kWh for the same weather can point to a refrigerant or airflow issue worth a service visit.
COP in dual-fuel decisions
In a dual-fuel system, the switchover point is where the heat pump's cost per unit of heat, based on COP and electricity price, equals the furnace's cost per unit, based on fuel price and furnace efficiency. Because COP falls as temperatures drop, there is usually a temperature below which the furnace becomes cheaper. Revisit that setting when energy prices change.
Reading a capacity table with COP in mind
When you compare models, look at heating capacity and COP (or HSPF2) at the same outdoor temperature listed on the AHRI or manufacturer table. A higher COP at 47°F means little if the unit you need must work at 5°F. Match the temperature column to your design low, then judge both output and efficiency in that row.
FAQ
Is COP the same as efficiency percent?
COP 1 equals 100 percent; COP 3 is like 300 percent.
Does cooling have a COP?
Yes; cooling uses EER or SEER2, which can be converted.
Can COP drop below 1?
Rarely, at extreme cold with heavy defrost, which is why backup exists.
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