Room air conditioners perform best when they are sized properly. It is essential to pick a unit with a cooling capacity that can handle the room where it is installed. A unit that is too small may be inadequate to cool a room, but an oversized air conditioner will cycle on and off, wasting energy, increasing electric bills, straining the unit, and doing a poor job of dehumidifying the room.

Before shopping, measure the dimensions of the room you want to cool. Multiply the length by the width to get the square footage. Then, when you choose an AC unit, match room size with the BTU rating, which should be printed on the packaging box. British Thermal Units (BTUs) measure the heat that an air conditioner can remove from a room.

Room air conditioners operate at 5,000–24,000 BTUs per hour. Most have sizing charts right on the box. Expect the size and cost of the air conditioner to increase as the BTU rating goes up. For example, an 8,000 BTU Fedders room air conditioner retails for about $170 whereas the 10,000-BTU LG window unit sells for about $250.

Small room air conditioners generally operate at 5,000–7,000 BTUs per hour and can adequately cool 100–300 square feet. Mid-sized models may run at 7,000–8,000 BTUs per hour. They can cool a room of up to 550 square feet.

Variables that can affect cooling capacity include:

• If you live in a very warm climate, you may need an AC that outputs more BTUs per hour than recommended.

• You may need a higher BTU rating for air conditioners located in kitchens and other rooms with a lot of heat.

• Portable air conditioners generally are not as efficient as window air conditioners, so it is a good idea to get a more powerful unit.