EER is an abbreviation for Energy Efficiency Rating. The Air-Conditioning and Refrigeration Institute standardized this rating, which reports central air conditioning efficiency at 80 degrees F indoors and 95 degrees F outdoors. This rating measures steady-state efficiency — that is, the efficiency of the air conditioner once it is up and running.
The SEER (Seasonal EER) rating is available for residential central air conditioners, and is generally considered a more reliable indicator of the overall energy efficiency of the unit than the EER.
While the SEER has been criticized for not taking into account the efficiency of the system while under partial load, there is no widely available rating which accounts for this issue in residential air conditioners. (However, the IPLV measures partial load efficiency in non-residential units.)
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