Seasonal Energy Efficiency Ratio. A measure of seasonal or annual efficiency of a central air conditioner or air conditioning heat pump. It takes into account the variations in temperature that can occur within a season and is the average number of BTU (British thermal units) of cooling delivered for every watt-hour of electricity used by the heat pump over a cooling season. The higher the SEER, the more efficient the unit. The more efficient the unit, the lower the operating cost. The U.S. Government’s minimum SEER rating is 15.