It might surprise you to know that buying a bigger room air-conditioning unit won't necessarily make you feel more comfortable during the hot summer months. In fact, a room air conditioner that's too big for the area it is supposed to cool will perform less efficiently and less effectively than a smaller, properly sized unit. This is because room units work better if they run for relatively long periods of time than if they are continually, switching off and on. Longer run times allow air conditioners to maintain a more constant room temperature. Running longer also allows them to remove a larger amount of moisture from the air, which lowers humidity and, more importantly, makes you feel more comfortable.
SEER is the Seasonal Energy Efficiency Rating. SEER rates the efficiency during the cooling season. Look for a SEER rating of 13 or above.
Evaporative coolers may be installed as an alternative to air conditioning, particularly in climates with very dry air. Evaporative coolers provide mechanical cooling to a building by either direct contact of air with water (direct evaporative cooler) or a combination of a first-stage heat exchanger to pre-cool the air and a second stage with direct air contact with water (indirect/direct evaporative cooler).
Source: U.S. DOE/Energy Star