EER stands for Energy Efficiency Rating.  It is a rating system to determining how efficient an air conditioner or heat pump is.  It simply uses a formula to determine how much power (electricity) it takes to product a certain amount of cooling (BTUs) at a specified set of conditions.

(Technically you can’t produce cooling, you actually are removing heat).

C0oling (BTUs) / Power (watts) = EER

Test Conditions: 95 degree outdoor temp, 80 degree indoor dry bulb temp and 67 degree indoor wet bulb temp