EER (Energy Efficiency Ratio) measures an air conditioner or heat pump’s cooling efficiency at a single outdoor temperature (typically 95 degrees F), unlike SEER which averages performance across an entire cooling season. EER is calculated by dividing cooling output (BTU/hr) by electrical input (watts).
EER is commonly used for commercial equipment, window/room air conditioners, and in climates with consistently high summer temperatures where peak-load efficiency matters more than seasonal average. A higher EER indicates greater efficiency at peak conditions.
As a rough conversion: EER is approximately equal to SEER multiplied by 0.875. The federal minimum EER for window AC units is 10.0 for units under 6,000 BTU/hr.