Both EER (Energy Efficiency Ratio) and SEER (Seasonal Energy Efficiency Ratio) are indicators of how efficient an air conditioner unit is. The higher the number, the more efficient it is and the lower the operating cost. SEER is used with central air conditioners, while EER is used with room air conditioners.
The current minimum requirement is 10.0 SEER. This is significantly more efficient than older units. Ratings of 11.0 to 12.0 are common and will have lower operating costs. Units with SEERs as high as 15 are available.
The more you use your central air conditioning, the more you will benefit from higher SEER ratings, and the more likely that the additional cost will be offset by energy savings.