Leave Your AC On Or Turn It Off?

A common myth about the way air conditioners work may be wasting your money and contributing to global warming.

July 21, 2011

We’ve all wondered this at some point, usually after coming home to a stifling house on a long, hot, summer day: Do you save energy by shutting off your air conditioner when you’re gone for the day? Or is it better to just let it run at the same temperature setting all day long, so you don’t expend extra kilowatts starting it up and getting the premises to a comfortable temperature? Wasting energy not only costs you more money, it leads to more burning of coal and other fossil fuels that contribute to global climate change. We contacted Jennifer Thorne Amann, MES, buildings program director for the American Council for an Energy-Efficient Economy (ACEEE), to help us figure it out.

THIS: Leave the AC running, set at the same temperature all day.


The theory: If you leave the AC running with the temperature set at a comfortable setting, the stable temp means the air conditioner won’t have to work as hard.

THAT: Turn the AC off when you leave; turn it on when you get home.

The theory: If you let the house get hot when you’re away, the air conditioner will use more energy when it’s time to cool things down.


That. Turn it off when you leave. It may seem like a waste of energy to turn your air conditioner on and off, but doing so actually saves you a fair amount of money and helps your air conditioner work more efficiently, says Amann. While it may seem like your unit has to work hard to cool a space down from 80 to 75 degrees, “air-conditioning systems run most efficiently when they’re running at full speed,” rather than running for shorter periods at a less powerful speed to maintain a constant temperature all day, Amman says. They’re also better able to dehumidify your house, she adds, when they run full blast. If you have central air, or a window unit with a thermostat, you can save energy and keep things from getting unbearably torrid by setting the thermostat higher. ACEEE estimates that air conditioners use 3 to 5 percent less energy for every degree you raise the thermostat. To get the best energy savings, leave your thermostat set at 78 degrees or higher while you’re out.

Here are a few other ways to stay cool and get better AC energy savings this summer:

  • Buy a programmable thermostat. Whether you turn your central air off or turn its thermostat up when you’re out of the house, a programmable thermostat can crank up the cool factor before you get home. That way your house will be comfortable when you walk in the door. Amann notes that you’ll need to buy a thermostat that suits both your cooling and your heating systems. Some models don’t work with heat pumps, which can pose a problem in the winter, so check with whoever installed or services your system. If you use a window unit, an appliance timer can serve a similar purpose; newer models may include a timer as well as a thermostat.
  • Buy a ceiling fan. Fans don’t necessarily cool a room, but they move air across your skin, making you feel comfortable at a higher temperature. That allows you to set the thermostat higher and stay cool while using less energy (especially if you buy a fan that’s Energy Star rated). However, because it doesn’t reduce the temperature of a room, it’s a waste of energy to leave a ceiling fan on in the hopes that it will keep your house cooler while you’re gone.
  • Plant some bushes. Large, shady bushes planted on the south and west sides of your house will reduce heat gain during the day.
  • Create crosswinds. The easiest way to cool a house? Open a window—but not too much. The less you open it, the more of a draft you’ll create. You can create cross breezes in a one-story house (or a single room) by cracking one window’s bottom sash and another’s top sash. If you live in a two-story house, open a window on the first floor a crack and another window upstairs on the opposite side of the house. Experiment to see which windows work best and how much you should open them.

 Courtesy of rodale.com

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