Heat pump COP: what it means and why it changes
COP is the simplest way to compare how efficiently a heat pump turns electricity into heat. Here is how to read it without being misled.
COP stands for coefficient of performance. It is the amount of heat a heat pump delivers divided by the electricity it uses to deliver it. A COP of 3 means three units of heat for every one unit of electrical energy.
That sounds impossible, but a heat pump does not make heat. It moves heat that already exists in the outdoor air, even on a cold day. The electricity only runs the compressor and fans.
How to read a COP number
A COP of 1 is the same as a simple electric resistance heater: one unit in, one unit out. Anything above 1 means the heat pump is doing better than that. The higher the number, the less electricity you need for the same warmth.
Keep one thing in mind. A COP is a snapshot at one set of conditions, not a promise for the whole winter.
Why COP drops when it gets colder
The colder the outdoor air, the harder the system works to pull heat out of it. The compressor has to lift the refrigerant across a bigger temperature gap, and that uses more electricity for the same output. So COP is highest on a mild day and lowest during a hard freeze.
This is why a single COP figure on a spec sheet can mislead you. Ask at what outdoor temperature it was measured.
| Outdoor conditions | What happens to COP | What it means for you |
|---|---|---|
| Mild day | Highest | Cheapest heating hours |
| Cold day | Lower | Still efficient on a well-sized system |
| Hard freeze | Lowest | Backup heat may run more |
COP, HSPF2 and SEER2: which one should you use?
COP is a single-moment measure. HSPF2 is a seasonal rating for heating that averages performance across a typical winter. SEER2 does the same for cooling. If you want to compare two models for heating, HSPF2 tells you more about a full season than one COP number does.
Use COP to understand the idea. Use HSPF2 to compare products. ENERGY STAR lists certified heat pumps, which is a fair place to start your shortlist.
What actually moves your real-world COP
- Correct sizing: an oversized unit cycles on and off, an undersized one leans on backup heat.
- Clean filters and coils, so air moves freely.
- Sealed, insulated ducts that do not leak heated air.
- A thermostat setup that does not force constant backup heat.
Our advice: do not chase the biggest number
A high COP on paper is worth little if the unit is the wrong size or the ducts leak. The best-rated model installed badly will lose to a decent one installed well. Compare seasonal ratings, then spend your effort on a good installer and on sizing. If your system seems to run constantly in winter, a technician can check whether something is dragging performance down.
Frequently asked questions
What is a good COP for a heat pump?
Anything above 1 beats electric resistance heat. Modern heat pumps commonly run well above that in mild weather, but the number falls as the outdoor temperature drops, so always check the conditions behind it.
Why does my heat pump COP drop in cold weather?
The system has to pull heat from colder air and lift it across a larger temperature gap. That takes more electricity for the same amount of heat, so efficiency falls.
Is COP the same as HSPF2?
No. COP is a measure at one set of conditions. HSPF2 is a seasonal rating that averages heating performance across a typical winter, which makes it better for comparing models.
Sources
What this page explains is based on these official sources. If you check one and it does not match what you read here, send it to us and the article gets corrected.
- ENERGY STAR. Air-Source Heat Pumps
- ENERGY STAR. Energy Savings at Home: Heating & Cooling