Leave It Running
The theory: If you leave the it running, the stable temperature means the air conditioner won't have to work as hard. 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
Turn It Off
The theory: Even if it has to work hard to cool the house in the evening, that doesn't waste more than letting the air conditioner run all day.
Amann's advice? Turn it off when you leave. It may seem like a waste of energy, but it actually saves you a fair amount of money and what's more, helps your air conditioner work more efficiently. 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 full blast, she adds.
And, 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, if you don't want to turn it off entirely, 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.
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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.