Here are a few terms you’ll come across when shopping for a window air conditioner, plus some other tips after our years of testing them. 

BTU stands for British Thermal Units. In the case of air conditioners, BTU is a way to measure how much heat the compressor can remove from a room. It’s a quick and easy way to figure out whether an AC unit is powerful enough to cool your space. First, you’ll want to find the square footage of your room by multiplying the length and width. Then, use the US Department of Energy’s guidance on the BTU capacity you need. For example, a 150 to 250-square-foot room needs a 6,000 BTU AC unit or higher for adequate cooling. 

Check the Combined Energy-Efficiency Ratio (CEER) rating. The specs on any air conditioner you buy should list a CEER rating, which is one of the best ways of checking the energy efficiency of a unit. You’ll usually see a number between 9 and 15. The higher the number, the less you’ll pay when the electricity bill comes around. A cheap window AC unit might save you money at first, but you may end up shelling out more in the long run. The US Energy Star program has a website that lets you browse AC units based on their CEER ratings. 

Check local laws. Some cities, like New York City, require installing brackets to support your window AC. A simple one like this model should do the trick, though we haven’t tried it out. You may also need to head to a hardware store for some plywood to make sure your window sill sits flat, but this depends on the type of windows you have and the AC model you buy. When installing, you should get a friend to help you out. These units can be heavy and difficult to hold, and the last thing you want is to drop one out the window. 

Measure your window. Before you buy, read up on the supported window types and sizes for the AC unit you’re looking at, and measure your window to be safe. Make sure to seal any gaps as best you can with the included foam (you can always buy more if you need it).

Correction July 28, 2021: We’ve clarified that BTU measures how much heat can be removed, not how much energy is needed for heat to be removed. Thanks, reader.