Heat pump COP calculator

What is COP and why it matters more than HSPF2 for heating decisions

Coefficient of performance is the most direct way to measure heat pump efficiency. It is the ratio of heat delivered to electricity consumed. A COP of 3 means the heat pump moves 3 kilowatt-hours of heat into your house for every 1 kilowatt-hour of electricity it draws. That is why heat pumps beat every other heating technology on operating cost: they move heat rather than create it, so they break the 100 percent efficiency ceiling that bounds combustion and electric resistance.

HSPF2 (Heating Seasonal Performance Factor v2) is a single number meant to summarize seasonal efficiency, but it averages performance across an entire heating season and hides what the heat pump actually does at the temperatures you care about most. COP gives you the snapshot at a specific outdoor temperature, which is what determines your bill on a cold January morning. Both numbers matter, but COP at your design temperature is the most honest predictor of cold-weather performance.

The COP formula and how it actually works

The math is one line: COP = useful heat output divided by electric energy input, in the same units. If your heat pump delivers 36,000 BTU per hour while drawing 3,000 watts, convert BTU to watts (1 BTU/hr equals 0.293 W) and divide: 36,000 × 0.293 = 10,548 watts of heat output, divided by 3,000 watts of electric input, equals a COP of 3.52. This means every watt of electricity you pay for delivers 3.52 watts of heat into your home. The other 2.52 watts come from outdoor air being compressed and cooled by the refrigerant cycle.

For cooling, the same equation applies but it gets a different name: EER (Energy Efficiency Ratio) in BTU per watt, or COP cooling if you express both sides in the same energy unit. A central AC with EER of 12 has a cooling COP of 12 ÷ 3.412 = 3.52, which is the same level of efficiency as the heat pump example above. Air conditioners and heat pumps use the same refrigerant cycle in different directions.

How outdoor temperature changes COP: the curve every homeowner should see

The single biggest factor on heat pump COP is the outdoor temperature. The colder the air outside, the harder the compressor works to extract heat from it, and the lower the COP gets. A standard air-source heat pump that hits COP 3.6 at 47°F (the AHRI rating point) drops to about 2.1 at 17°F and 1.5 at 5°F. Below 5°F, most standard heat pumps fall to a COP near 1.0, which is the same efficiency as electric resistance heat.

Cold-climate heat pumps, certified under the Energy Star Cold Climate spec, hold significantly better performance. A typical cold-climate unit hits COP 4.0 at 47°F, 2.8 at 17°F, and still delivers a useful 2.2 at 5°F. That is double the efficiency of a standard heat pump at sub-freezing temperatures, which is the entire reason the cold-climate category exists. If you live in a climate where winter design temperatures are below 20°F, the cold-climate premium pays back fast through avoided backup resistance heat.

Geothermal (ground-source) heat pumps are in a different league entirely. Because they draw heat from the ground (which stays at 50 to 60°F year-round in most of the US) rather than outdoor air, their COP stays in the 3.8 to 4.8 range regardless of how cold the outdoor air gets. The trade-off is upfront cost: geothermal installs run $20,000 to $40,000 because of the well drilling and loop field, vs $11,000 to $18,000 for a ducted air-source heat pump.

HSPF2 to COP conversion: the formula and what it really tells you

To convert HSPF2 to seasonal-average COP, multiply by 0.293. HSPF2 is rated in BTU per watt-hour, and 0.293 is the conversion factor to make the units cancel into a dimensionless ratio. An HSPF2 of 8.1 corresponds to a seasonal-average COP of 8.1 × 0.293 = 2.37. An HSPF2 of 9.5 (premium cold-climate territory) gives a seasonal-average COP of 2.78.

The catch with HSPF2-derived COP is that it is a season-long average across a defined heating climate region. Real performance at your design temperature can be significantly higher (at 47°F) or significantly lower (at 5°F) than the seasonal average. Use HSPF2 to compare equipment, but use temperature-specific COP to predict your actual heating bill on the coldest weeks. The DOE 2023 testing procedure (HSPF2) is more representative of real installations than the old HSPF, but it is still a seasonal aggregate, not a point measurement.

Carnot's theoretical maximum: how close real heat pumps get

There is a hard physical ceiling on how efficient any heat pump can possibly be, set by the second law of thermodynamics. The Carnot maximum COP for heating equals the absolute temperature of the hot side divided by the temperature difference between hot and cold. For a heat pump delivering 70°F indoor heat with 47°F outdoor air, the Carnot max is about 23. For the same heat pump at 5°F outdoor, the Carnot max drops to about 8.

Real-world heat pumps achieve 25 to 50 percent of the Carnot maximum for their operating conditions. The best inverter-driven cold-climate heat pumps and high-end geothermal units push past 50 percent of Carnot. The gap exists because of compressor inefficiency, heat exchanger losses, defrost cycles, and the energy spent moving air across the coils. Comparing your COP to the Carnot max is a useful sanity check: if a vendor quotes a COP over 70 percent of Carnot at any operating condition, the spec is almost certainly wrong or measured under cherry-picked conditions.

What counts as a good COP for residential heat pumps

Use these benchmarks when evaluating equipment:

• COP 4 or higher: excellent. Premium cold-climate or geothermal equipment running at moderate outdoor temperatures.
• COP 3.0 to 3.9: good. Mid-tier air-source equipment at 47°F, or cold-climate equipment at sub-freezing temperatures.
• COP 2.0 to 2.9: average. Standard air-source heat pump operating at cold but not extreme outdoor temperatures.
• COP 1.0 to 1.9: poor. Standard heat pump operating below its balance point, often with backup electric resistance kicking in.
• COP under 1.0: the heat pump is broken, in defrost, or the input numbers are wrong. Heat pumps cannot drop below 1.0 on their own; that would violate energy conservation.

Why COP at design temperature matters more than COP at 47°F

HVAC equipment is rated at 47°F outdoor temperature because the AHRI testing standard picked that as a moderate reference point. But the heating load on your home peaks at your design temperature, which is typically 0 to 25°F in cold climates and 25 to 35°F in moderate climates. The COP at your design temperature is what actually determines your January electric bill.

Use our heat pump sizing calculator to find your design temperature, then read the COP from the manufacturer's extended performance table at that temperature. Reputable manufacturers (Mitsubishi Hyper Heat, Daikin Aurora, Carrier Infinity Greenspeed, Bosch IDS Bond Premium) publish full performance tables down to -15°F. If a vendor cannot show you the COP at your design temperature, that is a red flag about the equipment they are selling.

How to use COP to compare heat pump quotes

When evaluating heat pump quotes, ask for three numbers: COP at 47°F, COP at 17°F, and rated heating capacity at 5°F. The first tells you mild-weather efficiency. The second tells you cold-weather efficiency. The third tells you whether the system can heat the house at all when it really matters. A heat pump with COP 3.5 at 47°F but COP 1.5 at 17°F will be expensive to run all winter. A heat pump with COP 4.0 at 47°F and COP 2.8 at 17°F is genuinely cold-climate capable. Use this calculator and our HSPF2 converter to cross-check the math behind any quote before signing.