SEER stands for Seasonal Energy Efficiency Ratio and it is the method used to rate the efficiency of your air conditioner. The higher the SEER the more efficient it is. How much more? For every SEER it is approximately 6% savings on your electrical usage. So when you upgrade from a 13 SEER to a 20 SEER unit you can save up to an additional 42% on your electrical usage.
In January 2006 the U.S. Government increased the minimum SEER rating to 13. Any equipment with less than this upgraded standard will become obsolete.