SEER

The efficiency of air conditioners is often rated by the Seasonal Energy Efficiency Ratio (SEER) which is defined by the Air Conditioning, Heating, and Refrigeration Institute in its 2008 standard AHRI 210/240, Performance Rating of Unitary Air-Conditioning and Air-Source Heat Pump Equipment. A similar standard is the European seasonal energy efficiency ratio (ESEER).

The SEER rating of a unit is the cooling output during a typical cooling-season divided by the total electric energy input during the same period. The higher the unit's SEER rating the more energy efficient it is. In the U.S., the SEER is the ratio of cooling in British thermal unit (BTU) to the energy consumed in watt-hours. The coefficient of performance (COP), a more universal unit-less measure of efficiency, is discussed in the following section.

For example, consider a 5,000-British-thermal-unit-per-hour (1,500 W) air-conditioning unit, with a SEER of 10 BTU/W·h, operating for a total of 1000 hours during an annual cooling season (e.g., 8 hours per day for 125 days).

The annual total cooling output would be:

With a SEER of 10 BTU/W·h, the annual electrical energy usage would be about:

The average power usage may also be calculated more simply by:

If your electricity cost is 20¢/kW·h, then your cost per operating hour is:

The Energy Efficiency Ratio (EER) of a particular cooling device is the ratio of output cooling energy (in BTU) to input electrical energy (in Wh) at a given operating point. EER is generally calculated using a 95 °F outside temp and an inside (actually return air) temp of 80 °F and 50% relative humidity.

The EER is related to the coefficient of performance (COP) commonly used in thermodynamics, with the primary difference being that the COP of a cooling device is unit-less, because the numerator and denominator are expressed in the same units. The EER uses mixed units, so it doesn't have an immediate physical sense and is obtained by multiplying the COP(or EER) by the conversion factor from BTU/h to Watts: EER = 3.41214 × COP (see British thermal unit).

...
Wikipedia