Spec sheets for whole-home cooling equipment quote several efficiency numbers that look similar but measure different things. CEER for room ACs. SEER and SEER2 for central systems. EER for single-condition performance under fixed test conditions. BTU for capacity. This guide covers what each measures, what's worth comparing, and how to use them in a purchase decision.
BTU — capacity
BTU/hr is the amount of heat a unit can remove per hour. It measures capacity, not efficiency. A 24,000 BTU unit can cool a larger room or faster than a 12,000 BTU unit, regardless of either's energy use.
Sizing by BTU matters first; an incorrectly-sized high-efficiency unit isn't more efficient than a correctly-sized mid-efficiency one. Read our guide on how to choose an efficient window AC for sizing.
EER — Energy Efficiency Ratio (single-condition)
EER = BTU/hr / watts input, under a specific test condition (95°F outside, 80°F inside, 50 percent relative humidity).
EER is a snapshot — single operating point, max load. It's the simplest efficiency rating and historically the standard, but it doesn't reflect real-world seasonal operating conditions. A high-EER unit that's only efficient at 95°F doesn't really save money in a climate that mostly runs at 85 to 90°F.
EER still matters in hotter regions where your cooling load spends a lot of time at high outdoor temperatures. For more temperate regions, SEER is the more representative number.
CEER — Combined Energy Efficiency Ratio (room ACs)
CEER was introduced specifically for room air conditioners. Unlike EER, it incorporates both the cooling efficiency and the standby/off-mode power the unit draws when not actively cooling.
Because window ACs spend a significant fraction of time in standby (waiting for the next call to cool), including standby draw gives a more realistic annual energy picture. CEER for room ACs typically runs 10 to 15; Energy Star qualification starts at 12+ for most size classes.
If you're shopping for a window unit, CEER is the rating to compare. Higher is better, with the next-integer-up CEER roughly meaning 7 percent less annual energy for the same cooling.
SEER / SEER2 — Seasonal Energy Efficiency Ratio (central systems)
SEER is the seasonal efficiency rating for central ACs and heat pumps, prescribed by AHRI test conditions across a range of outdoor temperatures.
SEER = total seasonal cooling output (BTU) / total seasonal electrical input (Wh).
Higher SEER = more cooling per unit of electricity across the cooling season.
As of January 2023, US federal minimums shifted to SEER2, which uses a higher external static pressure test condition. SEER2 numbers are roughly 4 to 7 percent lower than the older SEER numbers for the same equipment (so a 14 SEER unit is approximately a 13.4 SEER2 unit).
SEER2 minimums vary by region:
- Northern US: minimum 13.4 SEER2 (formerly 14 SEER)
- Southern US (SEER2): 14.0 SEER2
- Southwest (SEER2 + EER2 demand): additional lower-load efficiency requirement
Energy Star qualification in central ACs starts at 15.8 to 16.5 SEER2 depending on system type, with 'Most Efficient' tier qualifying at 18+ SEER2 for split systems.
HSPF2 — for heat pumps
Heat pumps also heat, not just cool. The heating-side efficiency rating is HSPF2 (Heating Seasonal Performance Factor 2), analogous to SEER2 but for heating output. Minimum HSPF2 in the US is 7.5 to 8.1 depending on region.
If you're shopping for a heat pump specifically, the HSPF2 number is what determines your winter electricity use. See our heat pump vs air conditioner guide for when a heat pump makes sense.
What's worth comparing
For room ACs: CEER. Compare within the same BTU bracket for a fair comparison.
For central ACs and heat pumps (cooling): SEER2. The minimum units you'll find in the market are 14 SEER2 (15 in southern US); high-efficiency units are 18 to 20+ SEER2.
For heat pumps (heating): HSPF2. High-efficiency units reach 10+ HSPF2.
For hot regions with peak-load concern: also check the EER2 at full load — modern equipment publishes both SEER2 and EER2.
For sizing: match the unit's capacity (BTU) to your climate and home's load calculation. An efficient unit that's the wrong size isn't efficient.
Practical guidance
A SEER2 bump from 16 to 20 typically reduces cooling electricity use by about 20 percent. Whether that's worth the price difference depends on:
- Annual cooling hours (more in hot climates)
- Cost of electricity
- Local labor and ductwork compatibility
For a Phoenix, AZ home with high cooling hours and high electricity rates, the extra cost of 20 SEER2 typically pays back in 5 to 8 years. For a Boston, MA home with shorter cooling seasons, the payback may exceed the unit's life unless state incentives offset the upfront premium.
The numbers themselves aren't enough — local labor costs, ductwork compatibility, and available deals matter too. Read the spec sheets, run the math for your state, and get multiple quotes.