Get the right capacity. Don't assume that bigger is better. Cooling is only half of an air conditioner's job; it also needs to remove moisture from the air. A unit that's too small for your room won't cool enough, but one that's too large won't have time to remove humidity effectively. So before shopping around, determine how many Btu of cooling capacity you need for your room.
Measure the window. Air conditioners vary in physical size. Even units with the same capacity in Btu can differ by several inches in their dimensions. Measure your window carefully while it's fully open to make sure the unit you're considering will fit. If your window opening is taller than it is wide -- if you have sliding or casement windows, for example -- look for a unit with a similar shape.
Consider window location. To effectively cool a room, an air conditioner needs to direct cold air to the center of the room. However, most air conditioners are better at blowing air in one direction than the other. Check the position of your windows and determine whether your air conditioner will need to direct air to the right or to the left.
Also, some air conditioners need extra space on one side or the other to provide access to the filter, so make sure you'll have enough room to install and remove the filter once the unit's in place. And, finally, measure the power cord -- it will need to reach the nearest outlet.
Check the electrical requirements. Most electrical circuits can handle up to 15 amps of electricity. That means that a large air conditioner, which can use up to 15 amps by itself, will need to be run on its own circuit to avoid tripping the circuit breaker. Smaller air conditioners may be able to share a circuit, depending on what else is plugged in with them.
Most window air conditioners, including all the models in this report, use a standard three-pronged plug, which can handle up to 125 volts and 15 amps. However, some extra-large models (over 12,000 Btu) may use 125 volts/20 amps, 250 volts/15 amps, or higher. If you choose one of these units, you'll need to call an electrician to have your circuits upgraded, which will add considerably to the cost of the air conditioner.