SEER stands for Seasonal Energy Efficiency Ratio and it measures (like gas mileage on a car) the electrical cost that individual units will incur from normal operation. Ten year old A/Cs have to at least be a 10 SEER or higher. The higher the SEER, the more efficient the unit. Effective January, 2006, the US Department of Energy mandated the minimum SEER for air conditioners in this region be 13. Effective, January, 2015, the US Department of Energy mandated the national minimum SEER for heat pumps be 14.