Too hot at home? It might be time for a new window air conditioner.
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Indoor air conditioning is one of the best ways to beat the summer heat. For many, the convenience of central air remains out of reach, which leaves the window AC unit as the next best thing.

Today’s window air conditioners are much better than what you might be used to, and nothing like the massive energy hogs they once were. Modern window units provide relief from searing temperatures, remove humidity and use less electricity than ever before.

However, if you don’t do your homework, it’s easy to end up with the wrong machine. You might get stuck with something too big, too small or that simply doesn’t fit where you want. Read on to learn how to avoid these pitfalls.

Read more: How to clean your window air conditioner

Know before you buy

Deciding on an air conditioner depends on a few key factors. Here is list of details you should figure out before you shop.

Target coverage area in square feet (or square meters)
Size and shape of your windows
Where to channel the water from condensation
Position and voltage of nearby electrical outlets
What smart features are available

Figure out your square footage

First you’ll want a good handle on what you need from your new AC unit. The most basic question: How well does it cool a room?

Window-unit air conditioners are all rated with British thermal units — usually ranging between 5,000 and 12,000. BTUs are a measure of energy that essentially tells you how effectively a device can lower the temperature in a given space.

For a small room (meaning 150 square feet or less), a 5,000 BTU model should work nicely. When it comes to slightly larger rooms — particularly ones in the range of 340 square feet — 8,000 BTU window units will be your best bet. If you have a large room or want to cool a small apartment with a single AC unit, you’ll need something with a little more power. 12,000-14,000 BTU units can cover square footage between 550 and 800.

The Environmental Protection Agency Energy Star website has a handy tool to help you determine roughly how many BTUs you’ll need.

Generally, though, you want 20 BTUs for every square foot you want to cool (though other factors, such as a vaulted ceiling or shade coverage, can push the final estimate up or down slightly).

Calculate the right size AC unit you need.
EPA Energy Star

What size are your windows?

Typically, window-mounted AC units are exactly that. The body of the appliance sits partially inside and partially outside of the window pane. A bracket or brace of some sort holds the air conditioner in place. This bracket also divides the inside from the outside, and hot air from cool air.

Since there’s no universal standard for window size, measuring your window dimensions is a must. Once you have that info, you can then compare it against the specifications of any potential new machine. This way you’ll hopefully avoid any nasty surprises when it’s time to install.

Will you remove the window unit at the end of each summer?

Decide if you plan to remove your new air conditioner at the end of summer or leave it in the window year-round. There are trade-offs to either approach. With a permanent setup, you don’t have to reinstall your air conditioner each year.

Even if your permanent AC mount is well-insulated, though, it won’t be as draft-free as a quality window that’s completely shut. That can be a problem if you live in a place with near- or below-freezing temperatures in the winter. If you live somewhere with mild winters, you should be just fine.

A portable air conditioner represents a third option. These free-standing appliances move easily from room to room since they come with wheels. You need to mount an exhaust hose assembly inside your window, but the hose hardware is lightweight and installs much more easily than whole in-window AC unit. If you’re not allowed to have window units in your apartment or if your windows simply won’t hold a traditional device because of their design, a portable air conditioner might be something to consider.

Read more: Window air conditioners vs. portable air conditioners

Consider condensation

One downside to air-conditioned comfort is condensation. Similar to dehumidifiers, AC units pull moisture from the air as they run. That collected water must go somewhere, usually pooling in an internal drip tray. Some AC models advise you to drill a small hole in them for drainage. You can then hook a drain hose there, to channel excess water toward wherever you want it to go.

Electricity and power

Decades ago, it wasn’t uncommon for old air conditioners to need high-voltage (240V) electrical outlets. Thanks to Energy Star certification, current models are a lot more efficient. They can operate at 120V, the default voltage for US home appliances.

It’s a good idea to check the situation in your particular home. A legacy, defunct AC unit may only have a 240V outlet nearby. Here’s another problem you might run into. The closest outlet is 120V, but it shares a circuit with other power-hungry appliances or electronics. In either scenario, you may have to have an electrician make repairs or add extra circuits.

Sweat the smart stuff

Features beyond size and cooling power might seem minor, but they can have a meaningful impact on how you use a window air conditioner. Smart features are appearing on more window air conditioners, giving you control through a mobile phone app or via voice commands.

Here are the key smart window AC features you might want to look for: scheduling functions, eco modes and Wi-Fi connection. Each of these elements let you personalize your experience with a window AC unit, whether by setting timers and remotely controlling the device or by tweaking the overall energy usage.

Smart speakers like Alexa and Google Assistant can help control some newer AC units.
Tyler Lizenby/CNET

A few final things you need to know

Today’s window air conditioners are different beasts from the ones you grew up with. They’re more efficient, more powerful and smarter. So the big question isn’t necessarily which device is best: It’s which device is best for you?

If seasonal allergies are a problem for you every spring like they are for me, maybe a built-in ionizer is more important than voice control. But if you’re planning to work from home more, given the coronavirus pandemic, maybe scheduling and remote control on your phone is the crucial feature.

Also consider noise. Many AC units can make unholy racket when running, so be sure to check the decibel rating of whichever models you’re considering.

Sold on a window AC unit but not sure where to buy one? You can usually find air conditioners in stores like Home Depot, Lowes and Walmart. Online, all of those stores — along with Amazon — offer fairly extensive catalogues of window units.