Why do window AC units use so much energy?

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I live alone in a small apartment. My electric bill is usually a lot $20/month. After I got AC installed it doubled and even tripled in some summer months. I’m trying to preserve it by turning it off when I’m not home.

Why is a small window unit so big on energy use?

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In addition to the points other have made, it might be more efficient to leave the A/C on, instead of turning it off when you’re not home. While it might be set to “cool,” it should only kick on whenever it starts getting over your target temperature, so it’s not running constantly. If you turn it off when you’re not gonna be home for like 9 hours, it’ll have to work harder when you turn it back on to get it back to that target temperature. It might use less energy if it you leave it on, and it runs periodically, each time trying to overcome a smaller temperature differential than that 9 hour temperature differential.

Air conditioning, in general, is very expensive. They’re inefficient due to thermodynamics and there’s no way around it. The best you can do is seal up your apartment tight to minimize air loss, and block out sources of heat from getting in.

Turn off your AC when you’re going to be out of the house and open your windows instead of running the AC at night if it’s cool outside. Running it all day while you’re gone is never the right choice. The other people in this thread are wrong. Even if you want to try and turn it to an efficiency argument, the AC unit will run better when it doesn’t start/stop every 15 minutes.

Window units are actually terribly inefficient. Because they are classified as an appliance the energy efficiency requirements are considerably less strict than with residential hvac systems. Additionally your apartment is presumably older and therefore not well insulated this adds a great deal of thermal load to the space. In general the best option is to leave the system on all the time. Turning the unit off causes the humidity to rise and the structure and its contents to increase in temperature.