Based on size alone, an air conditioner generally needs 20 British thermal units (Btu) for each square foot of living space. Other important factors to consider when selecting an air conditioner are room height, local climate, shading, and window size. Also look for a filter that slides out easily for regular cleaning, logically arranged controls, a digital readout for the thermostat setting, a built-in timer, and an Energy Star-rating for efficiency. Replacing an older unit with an EER rating of 5 with one with an EER of 10 can cut your energy costs in half, according to the DOE.