Buying too large a unit is not only expensive, it can increase discomfort by not removing enough humidity from the air, leaving you feeling cold and clammy. Determine the size of the area to be cooled, making sure to close off unused rooms or areas where cooling isn't necessary. As a rule of thumb, an air conditioner should have 15 - 20 Btus for each square foot of living space to be cooled. Keep in mind, however, that this formula can be affected by a number of factors, such as the amount of shade around your house, amount of insulation, or size of your family. The Department recommends buying a model with an EER of 10 or higher.
Location is a key factor in operating efficiency.
Ideally, window units should be placed in the middle of the area to be cooled and on the north or shady side of the house. Do not obstruct the free flow of air around the unit. You will be cool and at the same time save energy. Check the filter once a month and clean as needed, and set the fan speed at low or medium, if you have the option. Install a timer to automatically control operating times so that the conditioner doesn't run while you are away from home.