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By Mike Prospero published 21 July 22

Need a new window A/C? Read this first.

As the weather heats up, countless homeowners and apartment-dwellers will pull their air conditioners out from wherever they’ve been stored over the winter, and perform the bi-annual rite of wrestling them into a window. But, when you finally plug it in, waiting for that nice gust of cold air…nothing happens.

If you’re in the market for a new air conditioner this summer, it’s tempting to simply go out and buy the cheapest model your budget will allow. (You’ll want to check out our listing of the best air conditioner deals along the way). But before you make your choice, there are 3 things you should look for first. We’ll explain what each means, and why they’re important.

BTU stands for British Thermal Unit, and is used to measure the amount of heat that’s needed to raise or lower one pound of water by one degree Fahrenheit. The term is used for all sorts of appliances; if you’re buying a gas range, you’ll find BTU ratings for the burners, for instance. Basically, the higher the BTU number, the more powerful it is. A stove with a 10,000-BTU rating is going to be able to heat up a pot of water faster than a stove with a 1,000-BTU rating.

Similarly, an air conditioner with a 10,000 BTU rating will be able to cool a room down faster than an air conditioner with a 1,000 BTU rating. So, when buying a new air conditioner, you should get one with the highest BTU rating, right? Wrong.

When buying an air conditioner, it’s important to get one with a BTU rating that best matches the space you’re trying to cool. If you purchase an air conditioner with too low a BTU number, it’s going to spend much more time and energy trying to cool your room, which wastes electricity — and drives up your bill. If you buy an air conditioner that’s overly large for the space you’re trying to cool, it will cool the room before it’s able to remove the humidity, leaving you cold and clammy. 

This chart from Energy Star (opens in new tab) shows how many BTUs you need in terms of room size. To calculate your room size, multiply its length by its width to get the square feet. However, these ratings are based on rooms with an 8-foot ceiling height, so if your room has a higher ceiling, you’ll want to pick an air conditioner with a higher BTU output.

Most retailers will list both the BTU and the optimal square footage for a given air conditioner, making it pretty easy to find the model that’s best for you.

CEER stands for Combined Energy Efficiency Rating, which is an indicator of how efficient an air conditioner is at cooling. The higher the number, the less power it uses. You might also see the term SEER (Seasonal Energy Efficiency Rating) or simply EER, both of which describe the same thing. 

Both numbers are calculated by dividing the number of BTUs by the number of Watts used to power the device. For example, an 8,000-BTU air conditioner that draws 500 Watts of energy would have a CEER rating of 16 — which, incidentally, would be a very good number. On Energy Star’s site, the most efficient window air conditioner has a CEER rating of 15.7.

Online retailers don’t usually let you search for CEER numbers, but many do let you filter choices by those that are Energy Star-rated, which is a good place to start.

Air conditioners work by compressing a liquid refrigerant; that refrigerant is then released through a series of pipes. As the refrigerant decompresses, it gets colder, chilling the pipes it runs through. The air conditioner then uses a fan to blow air over those pipes and into your room, cooling it in the process. To compress the refrigerant, older air conditioners use what's called a rotary compressor, while newer models use an inverter-style compressor. 

If you can, look for air conditioners with an inverter-type compressor. Unlike a rotary compressor, which is either on or off, an inverter can ramp up and ramp down its power as needed. Not only is this more efficient — it uses less power in the process — but on the whole, it's also quieter.

It can be a little tricky to find out if an air conditioner uses a rotary or inverter compressor; you may have to dig around in the product manual.

How many people will be in the room?

The more bodies you have in a room, the warmer it’s going to be. (Cryptomining will also drive up your room’s temperature, too). While one to two people working in a home office won’t affect things much, any more, and you’ll want a more powerful air conditioner to cool things off.

Does the room face the Sun?

If the room you’re trying to cool has a southern-facing side, then chances are it will heat up faster than a room facing North. Energy Star’s guidance is that you should add 10% to your room size if it’s facing the Sun, and reduce it by 10% if it’s facing away from the Sun.

Consider a smart air conditioner

One advantage to buying one of the best smart air conditioners is that you can control them remotely from your smartphone. That way, you can make sure they’re off when you’re not home, so you’re not wasting energy. Smart air conditioners can also be connected to Alexa and Google Assistant, so that you can create automations around your home. For instance, you can create a routine to turn your air conditioner on automatically when you return home, and turn it off when you leave. 

Looking for other ways to stay cool? Here's how we used a $2 roll of aluminum foil to beat the heat. If you don't want to go that route, here are some other tips on how to cool a room down in a heatwave and 7 essential tips for staying cool in a heatwave.

Michael A. Prospero is the U.S. Editor-in-Chief for Tom’s Guide. He oversees all evergreen content as well as the Homes, Smart Home, and Fitness/Wearables categories, but also tests out the latest standing desks, webcams, drones, and electric scooters. He has worked at Tom's Guide for many a year; before that, he was the Reviews Editor for Laptop Magazine, a reporter at Fast Company, and, many eons back, an intern at George magazine. When he’s not testing out the latest running watch, electric scooter, or skiing or training for a marathon, he’s probably using the latest sous vide machine, smoker, or pizza oven, to the delight or chagrin of his family.

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