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SEER2, EER2, HSPF2 and COP explained

SEER2: seasonal cooling output (BTU) per watt-hour of electricity under DOE's 2023 test procedure. Higher = cheaper cooling.
EER2: cooling efficiency at a single hot test condition (95°F outdoor). Matters in very hot climates.
HSPF2: seasonal heating output per watt-hour. Higher = cheaper heating. Since electricity cost scales with 1/HSPF2, going from HSPF2 9 to 11 cuts heating kWh by about 18% for the same heat.
COP at 5°F: heat delivered ÷ electricity used at 5°F; a COP of 2 means 2 units of heat per unit of electricity (electric resistance heat is 1).
Heating capacity at 5°F/17°F/47°F: how much heat the unit can deliver at each outdoor temperature.

All are measured per 10 CFR 430 Appendix M1 and reported for each indoor/outdoor pairing (each has an AHRI reference number). ENERGY STAR's minimums and cold-climate rules: key product criteria.

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