Room air conditioners perform best when they are sized properly, so it is essential to pick a unit with a cooling capacity that is right for the room where it will be 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 air.

Before shopping, measure the dimensions of the room you want to cool. Multiply the length by the width to get the square footage. Then, match the 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; 12,500- BTU units are considered large. Expect the cost of the air conditioner to increase as the BTU rating goes up.

Small room air conditioners generally operate at 5,000–7,000 BTUs per hour and can adequately cool 100 to 300 square feet.

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

Mid-sized models run at 8,000–10,000 BTUs per hour. They can cool a room up to 450 square feet. A large unit rated between 10,000 and 12,500 BTUs will cool a room sized 400 to 650 square feet. See the chart below for complete recommendations.

Several variables can affect cooling capacity:

• If you live in a very warm climate, you may need an AC unit that outputs more BTUs per hour than recommended.
  
  
• Add 10% capacity for particularly sunny rooms or subtract 10% capacity for shady rooms.

• If the room you are cooling is permanently open to an adjoining space, figure the square footage of both rooms when calculating the size air conditioner you will need.
  
  
• Note: Portable air conditioners generally are not as efficient as window air conditioners, so it is a good idea to get a more powerful unit than the square footage indicates.