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The size window AC you need depends on the BTU and the size of the room you’re cooling. For a room with 8-foot ceilings, look for about 20 BTU per square foot of the room. For example, a 12-by-12-foot room should have an air conditioner with 2,880 BTU. You can buy something slightly more powerful, but don’t overshoot the BTU for a small room, or you’ll waste energy and have higher bills.

If possible, avoid placing furniture in front of a return air vent. A blocked air vent can cause your HVAC system to use more energy, which in turn can lower the life span of your unit and raise your energy bill. Furniture can also trap hot or cold air, damaging your furniture or appliances. 

Professional duct installers suggest leaving between six and 12 inches of space in front of an air return vent. If you must place furniture in front of a return air vent, try to aim for putting furniture over a floor vent instead of in front of a wall vent, and avoid draping curtains over any vent system.

Window AC units use a fair amount of electricity, but significantly less than central AC systems. Because they are more energy-efficient for cooling individual rooms or small spaces, their usage is comparatively low. On average, a window AC unit consumes between 500 and 1,400 watts, while a central AC system uses over 3,500 watts—meaning a window unit uses about one-third of the energy. This typically translates to a monthly running cost of $15 to $40, although this can vary depending on external factors.

To make the most out of your AC, consider implementing the following best practices: 

• Choose the right size AC for your home to ensure your unit runs as efficiently as possible.

• Choose a window with shade, as direct sunlight can cause your AC unit to overheat. 

• Clean or replace the filter at least every two months. 

• Inspect the AC’s fan and straighten it if necessary. 

• Discontinue use immediately if you suspect a malfunction, as running the AC while broken can lead to significant issues. 

• Turn off (or adjust the temperature) when you leave the house.