Is It Better To Leave Your A/C On All Day Or Turn It Off?

Air conditioning myths could be wasting energy and your money. Here's what you need to know.

July 1, 2016
thermostatPhotograph by Heymo/Shutterstock

We’ve all wondered this at some point, usually after coming home to a stifling house on a long, hot, summer day—do you really 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?

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.

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Related: What Southerners Taught Us About Keeping Houses Cool

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 when they run at full blast, she adds.

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 three to five 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.

Related: Signs You're Using Your Air Conditioner Wrong

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 and 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. 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 is to open a window, but not too much. The less you open it, the more of a draft you’ll create, and it can also be a great natural air freshener by filling your home with pleasant fresh air. 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.

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