For a typical air-source heat pump, cooling is usually more efficient than heating over a full season. Heating efficiency changes more because the system must pull heat from colder outdoor air. As the temperature falls, heating capacity and COP usually decline.
That does not make heat-pump heating inefficient. A modern heat pump can deliver up to three units of heat for each unit of electricity it uses. It can also use up to 75% less electricity for heating than electric resistance heating.
Heat Pump Heating vs. Cooling Efficiency
| Operating Mode | How the Heat Pump Works | Main Efficiency Rating | Effect of Outdoor Temperature |
|---|---|---|---|
| Heating | Moves heat from outdoor air into the home | HSPF2 and COP | Efficiency generally falls as outdoor air gets colder |
| Cooling | Moves heat from the home outdoors | SEER2 and EER2 | Efficiency generally falls as outdoor air gets hotter |
| Typical result | Both modes can be efficient | Compare each mode with its own rating | Cooling usually has the advantage over a full season |
The U.S. Department of Energy defines COP as the useful heating or cooling effect divided by electrical input. A COP of 3 means the system moves three units of heat for every unit of electricity it uses.
Why Heating Efficiency Falls in Cold Weather
Heating efficiency falls when an air-source heat pump must extract heat from very cold outdoor air. Cold air still contains heat, but less usable heat is available. The compressor then has to work harder to move enough heat indoors.
Cold weather can also cause:
- Lower heating capacity
- A lower COP
- Defrost cycles that use additional energy
- Auxiliary or electric-resistance heat
- A larger temperature difference between indoors and outdoors
The U.S. Department of Energy states that the capacity and efficiency of conventional air-source heat pumps decrease significantly as outdoor temperatures drop. Cold-climate models limit this drop with improved compressors, refrigerants and controls.
Why Cooling Often Uses Less Energy
Cooling often performs better because the heat pump removes heat from indoor air and rejects it outdoors. In mild or moderately hot weather, that job is often less demanding than extracting heat from very cold outdoor air.
Cooling also avoids some heating-related energy use, including defrost cycles and supplemental electric heat. Efficiency still falls during extreme heat, especially if outdoor temperatures are very high or the system has dirty coils, restricted airflow or leaky ducts.
Do Not Compare SEER2 and HSPF2 Directly
SEER2 and HSPF2 measure different types of performance:
- SEER2 measures seasonal cooling efficiency.
- HSPF2 measures seasonal heating efficiency.
- COP measures efficiency under a specific operating condition.
A heat pump with a SEER2 rating of 20 and an HSPF2 rating of 10 is not twice as efficient at cooling. The ratings use different test methods, operating conditions and seasonal calculations, so their numbers cannot be compared directly.
Which Rating Matters Most When Choosing a Heat Pump?
The most useful rating depends on your climate and how you will use the system.
In a Hot or Cooling-Dominated Climate
Prioritize:
- A high SEER2 rating
- Strong EER2 performance
- Good humidity control
- Proper system sizing
- Efficient ductwork or a well-designed ductless installation
In a Cold or Heating-Dominated Climate
Prioritize:
- A high HSPF2 rating
- Heating capacity at 17°F and 5°F
- COP at low temperatures
- An ENERGY STAR cold-climate designation
- A suitable backup-heating plan
ENERGY STAR cold-climate heat pumps are tested for low-temperature performance, including operation at 5°F. ENERGY STAR also recommends choosing a system designed for the local climate and considering backup heat when temperatures fall below the unit's most efficient operating range.
Is Heat-Pump Heating Still Efficient in Winter?
Yes. Heat-pump heating is usually much more efficient than electric resistance heating, even when outdoor temperatures are low. Comparing a heat pump with a gas furnace is more complicated. The result depends on local electricity and gas prices, furnace efficiency, heat-pump performance and how often backup heat runs.
A cold-climate heat pump can suit a home that needs substantial winter heating. A standard model may be enough in a mild climate, but choosing equipment based only on its cooling rating can lead to poor winter performance.
Bottom Line
Heat pumps are generally more efficient at cooling than heating across a normal season, but heating can still use much less electricity than electric resistance heat.
Choose based on the work the system will do most often:
- Cooling-dominated climate: Focus on SEER2 and EER2.
- Heating-dominated climate: Focus on HSPF2, low-temperature COP and heating capacity.
- Any climate: Do not compare SEER2 and HSPF2 directly, and make sure the contractor sizes the system for the outdoor temperatures your home experiences.