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Every heat pump sold in the United States carries a set of efficiency ratings certified under federal test procedures. Since January 1, 2023 those ratings have the number "2" on the end — SEER2, EER2 and HSPF2 — because the Department of Energy changed the test conditions to better reflect real ductwork. If you are comparing a quote from 2021 with one from today, the numbers are not on the same scale.
SEER2: seasonal cooling efficiency
SEER2 (Seasonal Energy Efficiency Ratio 2) is the cooling output in BTU divided by the electricity used in watt-hours, averaged over a simulated cooling season. A higher number means less electricity for the same amount of cooling. The federal minimum for a new split-system heat pump is 14.3 SEER2 in every region, and ENERGY STAR certification for ducted heat pumps starts at 15.2 SEER2.
The relationship with your bill is inverse, not linear. Going from 15.2 to 19 SEER2 cuts cooling electricity by about 20%, but going from 19 to 23 cuts it by only another 17%. The first steps up in efficiency save the most money.
EER2: efficiency on the hottest days
EER2 is measured at a single, hot outdoor condition (95°F). It matters most in hot-dry climates such as Arizona, Nevada and inland California, where the system spends long hours near its peak load. Two units with the same SEER2 can have quite different EER2 values; variable-speed systems usually win on SEER2, while some single-stage units hold up surprisingly well on EER2.
HSPF2: seasonal heating efficiency
HSPF2 (Heating Seasonal Performance Factor 2) is the heating equivalent of SEER2: total heat delivered over a heating season divided by electricity used, including the backup electric resistance heat the system needs on the coldest days. ENERGY STAR currently requires at least 7.8 HSPF2 for ducted split systems. Divide HSPF2 by 3.412 and you get a rough seasonal COP — an HSPF2 of 9.5 means the heat pump delivers about 2.8 units of heat for every unit of electricity, on average, over the season.
Rule of thumb: for heating, each additional point of HSPF2 cuts heating electricity by roughly 10–12% compared with an 8–9 HSPF2 baseline.
COP at 5°F: the cold-weather number
COP (Coefficient of Performance) is instantaneous efficiency at one condition. ENERGY STAR's cold-climate criteria require a COP of at least 1.75 at 5°F. A COP of 2.0 at 5°F means the unit still delivers twice as much heat as a space heater using the same electricity — even in deep cold. That is why cold-climate heat pumps are a credible primary heating system in Minnesota, Maine or Montana.
Why ratings come as a range
A heat pump's outdoor unit is certified together with specific indoor coils or air handlers. The same outdoor model can earn 15.2 SEER2 with one coil and 19.5 SEER2 with another. On each model page we show the lowest and highest certified SEER2 across all ENERGY STAR-listed pairings, and our cost estimates use the lowest value so you are never surprised on the downside. Ask your installer for the AHRI reference number of the exact pairing they are quoting.
Turning ratings into dollars
The simplest honest estimate multiplies the unit's capacity by the number of equivalent full-load hours your climate demands, divides by the efficiency rating, and multiplies by your electricity price. That is exactly what the calculator on every model page does — and you can override the price and the hours with your own numbers from a recent bill. Read the full method on our methodology page.
What the ratings don't tell you
- Installation quality. Field studies repeatedly find that refrigerant charge, airflow and duct leakage can cost more efficiency than the difference between two rating tiers.
- Sizing. An oversized single-stage unit short-cycles and dehumidifies poorly. A right-sized variable-speed unit runs long, quiet and efficient.
- Controls. Aggressive thermostat setbacks can trigger backup resistance heat in winter and wipe out savings.