Vintage toned window with external air conditioner unit, Harlem, USA.

How to Size a Window Air Conditioner

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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.

How to test window ACs.

After installing a window air conditioner in a double-hung window in our lab, 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 can cool the room in less than 15 minutes.

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 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.

Match an AC to Your Room Size

Bedroom or Office (100 to 300 Square Feet)
For a small bedroom, home office, or guest room, look for an air conditioner with a capacity of 5,000 to 6,500 Btu. You’ll want one with good scores for comfort and quiet.

Master Bedroom or Playroom (250 to 400 Square Feet)
To cool a bigger or busier room, you’ll want to step up to a midsized air conditioner rated at 7,000 to 8,500 Btu. If it’s for a bedroom, heed our noise scores.

Living Room or Family Room (350 to 650 Square Feet)
For a living room or family room, you’ll want a large air conditioner, 9,800 to 12,500 Btu, especially if you have an open floor plan.

The Best Fit for Your Budget

Tougher federal energy standards for window air conditioners make newer models cheaper to run. Look for a unit with an Energy Star label and an energy-efficiency ratio of 10 or above. The higher the EER, the lower your operating costs. All the models in CR’s air-conditioner ratings have an EER of 10 or above.

Keeping your AC in peak condition can also help you save. Look for models with a filter that’s easy to remove for regular cleaning—a dirty filter causes an air conditioner to work harder.

Many air conditioners have a timer that you can set to cool the room before you get home rather than running it while you’re away.

And some of the newer connected models have apps that enable you to adjust settings remotely from your smartphone.


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