The type and size of air conditioner you need depends on your climate and cooling loads.
Evaporative coolers are practical in hot, arid regions such as the southwest.
When you are shopping for a central air conditioner, look for a SEER rating higher than 12.0. If you already have a forced-air heating system, you may be able to tie an air conditioner into existing ducts, depending on their size and your home's relative heating and cooling loads. A good HVAC contractor can do the calculations for you. Proper sizing and installation are key elements in determining air conditioner efficiency. Too large a unit will not adequately remove humidity.