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 packing 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, though 12,500-BTU units are considered large. Most have sizing charts right on the box that explain the size of room they can cool effectively. Expect the size and cost of the air conditioner to increase as the BTU rating goes up.

Why we like it: • 8,000 BTUs--great for mid-sized rooms • Remote control • Energy Star rating

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 run at 7,000–8,200 BTUs per hour. They can cool a room up to 450 square feet. A large unit rated 9,800–12,500 BTUs will cool a room sized 350–650 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.