SEER2, HSPF2 and COP: Reading Heat Pump Efficiency Ratings
The three numbers you will see
SEER2 (seasonal energy efficiency ratio) measures cooling: the cooling delivered over a typical season divided by the electricity used, in Btu per watt-hour. HSPF2 (heating seasonal performance factor) is the heating equivalent, also in Btu per watt-hour. COP (coefficient of performance) is heat delivered divided by electricity used, in the same units, so a COP of 3 means three units of heat for each unit of electricity. Higher is better for all three.
Why the "2" ratings look lower
Since January 1, 2023, US efficiency ratings use an updated Department of Energy test procedure that better reflects real ductwork by testing at higher airflow resistance. The same equipment scores lower under the new test, so a system once rated 15 SEER and 8.8 HSPF is rated about 14.3 SEER2 and 7.5 HSPF2. That is why the federal minimum for split-system heat pumps is now written as 14.3 SEER2 and 7.5 HSPF2. When comparing quotes, compare SEER2 with SEER2, never with an older SEER figure.
From HSPF2 to a rough efficiency
Because HSPF2 is in Btu per watt-hour and one watt-hour is 3.412 Btu, dividing HSPF2 by 3.412 gives an average heating COP under the rating's standard test conditions: 7.5 HSPF2 is roughly a COP of 2.2, and 9.5 HSPF2 roughly 2.8. Real seasonal efficiency depends on your climate, which is why the city pages on this site estimate a seasonal COP from each city's design temperature instead of using one rating for everywhere. In cold climates, the capacity and COP at 5°F tell you more than HSPF2 does; see the cold-climate units guide.
How much a higher rating is worth
Running cost scales roughly with the inverse of efficiency: going from 15 to 18 SEER2 cuts cooling electricity by about a sixth, not by 20%. Whether that pays for a more expensive unit depends on how much you heat and cool and what you pay for electricity, which is what the running-cost section of your city's page is built from. In mild, cheap-electricity places the premium for top-tier ratings often takes a long time to recover; in hot or cold places with expensive power it recovers faster. The guide to getting quotes covers how to price that difference.