EER and SEER both rate how efficiently an air conditioner cools, but they measure different things. EER (Energy Efficiency Ratio) is the efficiency on one hot day. SEER (Seasonal Energy Efficiency Ratio) is the average efficiency across a full cooling season. Higher is better on both, but the two scales do not line up.
The right number depends on your climate. EER is tested at a steady 95°F outside with 80°F indoors, so it shows how a system holds up when it works hardest. SEER mixes in cooler hours too, testing across 65°F to 104°F, so it rewards systems that sip power at part load. In a mild summer, SEER tells the better story; in relentless heat, EER does.
Since 2023 both use stricter tests and gained a 2 in the name: EER2 and SEER2. A system can post a high SEER2 yet a mediocre EER2 if it fades on the hottest days. That is why hot, dry Southwest states (Arizona, Nevada, New Mexico, and parts of California) set a minimum EER2 on top of the SEER2 floor — a seasonal average alone would hide weak peak-day performance.
For a buyer in a hot climate, check both numbers, not just the headline SEER2. ENERGY STAR sets its bar at EER2 of 12.0 or higher for central air conditioners and 11.0 or higher for heat pumps. And never compare an EER number to a SEER number — they sit on different scales, so a 12 of one is nothing like a 12 of the other.