SEER stands for "Seasonal Energy Efficiency Ratio. This is a measure of the energy efficiency of the air conditioning system. SEER ratings permit consumers to compare operating costs of various cooling systems and products.
SEER = [Total Cooling Output Over the Cooling Season] / [Total Electrical Energy Input Over the Cooling Season]
Higher air conditioning SEER rating means more efficient, or in other words lower energy cost to cool the building.
Older air conditioning systems are likely to have a lower SEER (perhaps 5 or 6) than a newer more efficient system (SEER of 13 or higher). But beyond comparing SEER ratings, a look at the building insulation, air leakage, and the layout, insulation, and adequacy of the air conditioning duct system are likely to have a very large, usually determining effect, on the operating cost of air conditioning systems in buildings.