Buying a bigger room air conditioner 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. The reason: an oversized unit will cool the room(s) to the thermostat set-point before proper dehumidification occurs, making the area feel “clammy” and uncomfortable. Central air-conditioning systems need to be sized by professionals. Explore our Energy Saver 101 infographic on home cooling to learn how an air conditioner works.
If you have a central air system in your home, set the fan to shut off at the same time as the compressor, which is usually done by setting the "auto" mode on the fan setting. In other words, don't use the system's central fan to provide air circulation -- use circulating fans in individual rooms.
Instead of air-conditioning, consider installing a whole-house fan. Whole-house fans work in many climates and help cool your home by pulling cool air through the house and exhausting warm air through the attic. Use the fan most effectively to cool down your house during cooler times of the day: your home will stay cooler through the hotter times of the day without using the fan.
U.S. Department of Energy - Energy Efficiency and Renewable Energy