Both are energy efficiency ratings that quickly allow you to compare different models. EER is an acronym for Energy Efficiency Ratio, SEER is an acronym for Seasonal Energy Efficiency Ratio. EER’s are used to rate efficiency levels of packaged air conditioners, SEER’s are used to rate efficiency levels of split systems (indoor unit piped to outdoor unit). The higher the value, the higher the efficiency. Do NOT compare an EER value against a SEER value. These 2 ratings use different testing procedures.