The “seasonal” in Seasonal Energy Efficient Ratio (SEER) plays a big part in how high a rating you may need to cool your home comfortably. An AC unit installed in Connecticut, say, might run for a couple of months out of the entire year with a minimum number of days above 90 degrees. An AC unit intended to cool a southwestern home may run upwards of six months of the year with regular summer temperatures regularly exceeding 100 degrees. The regulation is that all units have a minimum SEER rating of 13. In sunbelt regions such as the southwest, a rating of 16 or higher might be the best choice.
While units have been developed that earn a SEER rating of 30, that number is excessively high for most installation purposes.