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.
Set your thermostat at as high a temperature as comfortably possible in the summer, and ensure humidity control if needed. The smaller the difference between the indoor and outdoor temperatures, the lower your overall cooling bill will be.
Avoid setting your thermostat at a colder setting than normal when you turn on your air conditioner. It will not cool your home any faster and could result in excessive cooling and, therefore, unnecessary expense.
Consider using an interior fan along with your window air conditioner to spread the cooled air through your home without greatly increasing your power use.
Avoid placing appliances that give off heat such as lamps or TVs near a thermostat.
LONG-TERM SAVINGS TIPS
If your air conditioner is old, consider buying an energy-efficient model. Look for the ENERGY STAR® and EnergyGuide labels -- qualified room air conditioners are 10% more efficient, and qualified central units are about 15% more efficient than standard models.
Consider installing a whole-house fan or evaporative cooler if appropriate for your climate.
U.S. Department of Energy - Energy Efficiency and Renewable Energy