The coefficient of performance and the energy efficiency ratio measure the efficiency of air conditioners and heat pumps. Heat pumps are rated by EER when used as an air conditioner and COP when used for heat. The higher the EER and COP, the more efficient the system. COP is a unitless measure. It is the energy acquired or heat produced divided by the energy applied. The EER is expressed in British thermal units per watthour. It is the cooling capacity divided by the energy applied. To convert from EER to COP, divide by the conversion factor 3.413; to convert from COP to EER, multiply by 3.413.

Get the EER. It should be on the cooling system's yellow EnergyGuide label, which manufacturers are required to display on many appliances. This label lists how much energy the appliance uses, compares the energy use of similar products and indicates the approximate annual operating costs. According to the U.S. Department of Energy, national appliance standards require room air conditioners to have an EER in the 8.0 to 9.8 or greater range, depending on the type and capacity.

Convert the EER to COP. Divide the EER by 3.413 to get COP. For example, if the EER is 4.5 Btu/watthour, then the COP is about 1.32, the result of 4.5 divided by 3.413.

Multiply the COP by 3.413 to get the EER. For example, if the COP is 1.5, then the EER is about 5.12 Btu/watthour because that is the total of 1.5 times 3.413.
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