Compare them carefully, because the labels are not interchangeable. SEER2 (Seasonal Energy Efficiency Ratio 2) is the current seasonal cooling metric for central air conditioners and heat pumps, including most mini-splits, under 10 CFR 430 Appendix M1 — the same family of tests this site already cites for HSPF2. EER or EER2 is efficiency at a single, hot operating condition, also in Btu per watt-hour, and is useful for a peak afternoon but is not a season. CEER (Combined Energy Efficiency Ratio) is the metric ENERGY STAR and DOE use for room air conditioners: cooling output divided by energy use that includes standby or off-mode power, in Btu per watt-hour. A window unit’s CEER and a mini-split’s SEER2 should not be ranked as “22 is better than 15” without checking that they are the same kind of test, because they are not.
For operating cost, the honest comparison is estimated seasonal kWh for the room you are cooling, times your electricity price — not a trophy efficiency number. A right-sized, well-installed mini-split is often more efficient in use than a window unit, and it can also heat; that still has to be shown with energy, not with mixed labels. The ductless-versus-window page is the companion to this one.
ENERGY STAR’s heat-pump key-product criteria (retrieved 2026-11-11) and its room-air-conditioner criteria are the definitions this site relies on. If a brochure only prints an older EER or a pre-SEER2 “SEER,” do not mix it with a SEER2 number. The calculator is built for heating comparisons (HSPF2 vs AFUE) and will not convert CEER into HSPF2.
This is not a permit calculation, a utility approval, or a substitute for a licensed electrician, HVAC contractor, or the authority having jurisdiction. Panel Headroom is an independent educational site. It is not a utility, a certification body, or a contractor. Electrical and generator work that is done incorrectly can cause fire, shock, or carbon monoxide poisoning. Use the adopted electrical code and manufacturer instructions; pull permits where required.