How to Size a Window Air Conditioner

Size matters when you’re buying a window air conditioner. An AC that’s too small will struggle to keep the room at a comfortable temperature; a model that’s too big will cool the room too quickly without removing enough humidity from the air.

Choose just right and you’ll feel just right—and save money, too. Consumer Reports tests air conditioners in rooms that are the same size as the ones they’re intended to cool. That makes it easier for you to select the best model for your needs.

How we test window ACs. After installing a window air conditioner in a double-hung window in our lab, we crank the heat up to 90° F in the surrounding area and measure how long it takes the AC to cool the room by 10° F.

“The best models in our tests can cool the room in less than 15 minutes,” says Chris Regan, the engineer who oversees CR’s air-conditioner tests.

We also gauge how accurately the AC reaches the set temperature, whether each model can recover after a brownout, how intuitive the controls are, and how loud each unit is on its lowest and highest setting.

The Rules for Keeping Cool
Window air conditioners typically have a cooling capacity ranging from 5,000 to 12,500 British thermal units (Btu/hr.). As a rule of thumb, an air conditioner needs 20 Btu for each square foot of living space.

But don’t buy by Btu alone. Other considerations, such as the ceiling height and the size of your windows and doorways, might call for for more cooling power.

To measure your room, multiply the length of the room by the width. Add together the size of rooms that aren’t separated by doors because the air conditioner will need to cool both spaces. Energy Star recommends that you make adjustments for the following circumstances:

• If the room is heavily shaded, reduce capacity by 10 percent.
• If the room is very sunny, increase capacity by 10 percent.
• If more than two people regularly occupy the room, add 600 Btu for each additional person.
• If the unit is used in a kitchen, increase capacity by 4,000 Btu.

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