EER2 (Energy Efficiency Ratio 2) measures cooling efficiency at a single design-day condition: 95°F outdoor temperature, 80°F indoor temperature at 50% RH. Higher = more efficient at peak load.
SEER2 is a seasonal average across mild and hot conditions; EER2 captures performance only at peak. In Charleston, where summer afternoons regularly hit 95°F with high humidity, EER2 is arguably more meaningful than SEER2 for predicting actual peak-load efficiency.
EER2 isn't the headline number on most equipment spec sheets but is published — typically buried in the AHRI listing. For homes with utility programs that bill on peak demand, EER2 matters more than SEER2.
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