When the air conditioning unit is properly matched with a furnace or air handler, you get maximum efficiency and longer system life. The cooling efficiency is measured using a Seasonal Energy Efficiency Ratio (SEER). A higher SEER rating means higher energy efficiency.
The typical spread is from 13 SEER (the Department of Energy minimum requirement) to just over 25 SEER.
High SEER ratings tend to mean a higher cost for the AC unit, but also can mean a higher quality. When you see a unit’s rating it's best to compare the cost of a higher-SEER unit with the amount of money you’d actually be saving on your electric bill.