EER (Energy Efficiency Ratio) measures an air conditioner's cooling efficiency at a fixed operating point: 95 degrees F outdoor temperature, 80 degrees F indoor temperature, and 50% relative humidity. It's calculated by dividing the cooling output in BTU per hour by the electrical input in watts. A window AC with 12 EER delivers 12 BTU of cooling per watt, using about 17% less electricity than a 10 EER unit of the same capacity.
While SEER averages efficiency over a whole season, EER gives you a snapshot of performance at peak conditions. Think of it as your AC's "top speed" efficiency — how well it performs when the outdoor heat is at its worst and the system is running full blast.
The EER Formula
EER = Cooling Capacity (BTU/h) / Power Input (W)
For a window AC rated at 8,000 BTU/h that draws 667 watts at the test condition:
EER = 8,000 / 667 = 12.0
This means you get 12 BTU of cooling for every watt-hour of electricity at 95 degrees F outdoor temperature.
The formula is simple because EER measures a single operating point, unlike SEER which requires seasonal weighting calculations.
EER vs EER2: The 2023 Update
EER was replaced by EER2 on January 1, 2023, for newly manufactured equipment. EER2 uses the same 95 degrees F test condition but with higher static pressure (0.5 in. w.c. instead of 0.1 in. w.c.). EER2 ratings are approximately 4.7% lower than EER for the same unit.
If you're comparing an older unit rated in EER to a new unit rated in EER2, divide the old EER by 1.047 to approximate the EER2 equivalent.
Typical EER Ranges
Mini-splits tend to have the highest EER ratings because they have no duct losses and use highly efficient inverter compressors optimized for their specific indoor units.
When EER Matters Most
EER is most relevant in three situations:
Hot climates with sustained high temperatures. In Phoenix, Las Vegas, or Houston, your AC runs at or near full capacity for weeks. During these periods, EER (not SEER) determines your electricity consumption. A system with high SEER but mediocre EER saves money during shoulder seasons but not during peak summer.
Window and portable ACs. For room air conditioners, EER (now replaced by CEER) has historically been the primary efficiency metric because these units typically run at full capacity whenever they're on. There's less part-load operation to capture with a seasonal metric.
Comparing across different types of equipment. EER provides an apples-to-apples comparison between central systems, mini-splits, window units, and portable ACs at the same operating condition. SEER comparisons across equipment types can be misleading because seasonal testing assumptions differ.
EER to COP Conversion
To convert EER to the universal COP metric:
COP = EER / 3.412
Examples: 10 EER = COP 2.93. 12 EER = COP 3.52. 14 EER = COP 4.10.
This tells you the system removes 2.93 to 4.10 units of heat for every unit of electricity consumed at the test condition.
Historical EER Minimums
Limitations of EER
EER has several limitations that you should understand:
Single temperature snapshot. EER only tells you efficiency at 95 degrees F. If your climate rarely hits 95, the actual performance you experience most of the time will be better than the EER rating suggests.
Doesn't capture part-load efficiency. Variable-speed systems that modulate capacity can't showcase their efficiency advantage in a single full-load test. This is why SEER was developed — to capture the seasonal part-load benefit.
Doesn't include standby power. For window and portable ACs, the original EER didn't account for electricity used when the unit is plugged in but not actively cooling. CEER (Combined EER) was created to address this.
Indoor conditions fixed. The test assumes 80 degrees F / 50% RH indoors. If your thermostat is set to 72 degrees F, the actual operating conditions differ from the test, though the ranking between units remains similar.
Key Takeaways
- EER = BTU/h of cooling divided by watts of electricity at 95 degrees F outdoor temperature
- EER was replaced by EER2 in January 2023 (roughly 4.7% lower due to tougher testing)
- Good EER for central ACs is 11.5+, excellent is 13+. Mini-splits can reach 15+.
- EER matters most in hot climates where the AC runs at full capacity for extended periods
- EER is a single-point measurement — for annual cost estimates, SEER2 is more useful
- COP = EER / 3.412 — this converts to the universal efficiency ratio
- Window/portable ACs now use CEER (includes standby power) instead of EER
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