Energy efficiency means how well a system converts the elctrical input into a desired output. In case of central air conditioners it is the cool comfort. For central air conditioners, energy efficiency is expressed with Seasonal Energy Efficiency Rating or SEER. The SEER rating is an important aspect to look for when purcahsing a central unit. It is prominently displayed on every unit. The SEER rating is basically the ratio between the amount of electricity consumed in watts and the BTUs of heat energy extracted from the air over a given period of time.