E.E.R. and S.E.E.R. are two ways to determine the cost of operation of an HVAC unit. A 6 E.E.R. was the typical unit efficiency in 1974, and approximately 8 S.E.E.R. was predominant in the mid 80's. 10 S.E.E.R. became the first federally mandated minimum efficiency in 1992 for residential split systems and 9.7 S.E.E.R. for single package systems in 1993. Prior to 1992, individual states were free to establish their own individual minimum efficiency standards for air conditioners as well as other types of appliances. The new federal efficiency standards preempt any state standards and therefore we now have uniform efficiency standards for the entire country as opposed to a various array of different individual state standards. The minimum efficiency will increase to 12 S.E.E.R. in the US effective on Jan 23, 2006.