EER (Energy Efficiency Ratio) is a measure of how efficiently a cooling system will operate when the outdoor temperature is at a specific level. The higher the EER, the more efficient the system. It is calculated by dividing the cooling capacity in BTUs per hour (BTUH) by the power input in watts. EER is expressed as BTUH/watts.
CEER (Combined Energy Efficiency Ratio) is the new standard by which Window Air conditioners are rated by the DOE (Department of Energy), as of June 2014. It takes in account the energy used while the air conditioner is running, as well as the standby power used when the unit is not running but is powered on.
SEER (Seasonal Energy Efficiency Ratio) is most commonly used to measure the efficiency of a central air conditioner. It also measures air conditioning and heat pump efficiency, which is calculated by the cooling output for a typical cooling season divided by the total electric energy input during the same time frame. A higher SEER rating means greater energy efficiency. Current U.S. Government's minimum rating is 13 SEER.
Note: There is no industry standard for energy consumption for portable air conditioners.