It’s Actually Okay to Run the AC All Day, Even If You’re Not Home | Reviews by Wirecutter

Mar 14, 2025

By Thom Dunn

Thom Dunn is a writer focusing on home heating and cooling. He once blew up a power strip with a space heater and a Marshall half-stack.

During my childhood summers, leaving the house was a process: Turn off the AC, dash to the door, and close it as quickly as possible to minimize the loss of cooled air.

Those old energy-saving habits can be hard to break—I’ve only recently accepted that it’s okay to leave the lights on when I’m not in a room.

It got me wondering: Is it really worth it to shut off the AC when you leave the house?

I’ve been testing at least a dozen AC units every year since 2019, and air-conditioner efficiency has come a long way since the summer when ska was popular. So I ran some tests.

As it turns out, the most efficient way to keep your home cool and the bills low is to leave the AC running—ideally, with the temperature turned up toward the upper 70s.

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To figure out the most efficient way to run our AC, we set up two of the same units (a 12,000-BTU version of the Midea U, our pick) on two different floors of the same apartment building. We also took measurements from a third apartment, on a separate floor, equipped with a heat pump (an 18,000-BTU LG LAN180HYV3).

The heat pump and one window unit were in apartments that were roughly equal in size—about 1,200 square feet—with windows facing south. The other Midea window unit went into a third apartment, on the top floor, and it was about two-thirds that size, with windows facing west. The variations in elevation and sunlight direction would obviously affect the test results, but I also thought it would be helpful to compare the energy use in homes with different thermal envelopes under the same weather conditions. (Pro-tip: Insulation and weatherization make a huge difference!)

On the first day of our tests, we shut the AC units off at noon, and we turned them back on at 5 p.m., after the sun had passed its apex over the building. We recorded the temperature inside each apartment before we turned the AC back on, and then we tracked how long it took to get the indoor temperature back down to 72 degrees Fahrenheit.

The next day, we did the same thing—only this time, we left the AC running all day at 72 degrees. We used an energy monitor to track how much electricity each AC unit used over the course of each 24-hour period.

On both days, the outside temperature reached a whopping 93 degrees Fahrenheit.

To our surprise, all three apartments ended up using more energy when the AC was turned off than when we left it running all day. The first-floor unit, with the 18,000-BTU heat pump, used 7 kWh of energy on the day we turned off the AC, and it used only 4 kWh when we left it running. Using the average electricity rate in Massachusetts, this would be a cost difference of about 44¢ per day.

Similarly, the second-floor apartment, with a 12,000-BTU window AC unit, used 18 kWh when we turned the AC off and 12 kWh when we kept it on—a price difference of about $1.27 per day.

The westward-facing unit in the third-floor apartment—which was smaller than the other two apartments but also a better size for its 12,000-BTU window AC unit—saw the smallest change in energy use: 12 kWh on the first day with the AC unit off and 11 kWh when the unit was kept running. That might seem like a small difference, but it could still save you about 20¢ per day.

This test not only showed us how to save energy but also how to keep our homes more comfortable. On the day we turned the AC units off, the temperature inside each apartment got up into the low 80s; even after we turned the AC units back on, it still took another 90 minutes to cool the apartments back down again. In fact, only the apartment with the 18,000-BTU heat pump was able to get all the way back down to 72 degrees Fahrenheit before sunset. By contrast, the temperature in the two apartments with window AC units hovered around 75 degrees until several hours after dark, when the ambient temperature outside dipped as well.

If you’re going to have to wait that long to cool your home back down after shutting off the AC, you shouldn’t also have to deal with the higher utility bills that come along with it. Sure, you might be able to schedule the unit to turn back on with enough time to cool your home back down before your return. But then you have to do more work, and it’s still going to cost you more money than if you had just kept the AC running in the first place.

The AC units we used for these tests (like nearly all of the window units we recommend) use a variable speed inverter compressor. Unlike the air conditioners I grew up with, which really had only an “on” or an “off” mode, these newer models can fluctuate their energy usage based on how much cooling power they actually need at any given moment.

This is great for efficiency because these models use a lot less energy to maintain a steady temperature than they do to actively cool something down. And that’s precisely what we saw in our testing in a third trial, in which we left the air conditioners running all day but set the temperature to 78 degrees Fahrenheit, instead of 72 degrees.

This was the most efficient approach, and it led us to conclude the following: If you want to save energy without sacrificing much comfort, the best thing to do is to leave the AC unit running, but set its internal thermostat a little higher.

Those six degrees made a serious difference. The smaller apartment with the window air conditioner used only a quarter of energy this time—3 kWh instead of 11 kWh, or an average utility savings of about $1.70 per day. Similarly, the apartment with the 18,000-BTU heat pump needed only about 2 kWh to maintain a 78-degree Fahrenheit temperature, as opposed to the 4 kWh it took to run at 72 degrees. That would cost me less than 50¢ per day.

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We’re not the only AC testers who’ve gotten these kinds of results. According to NYSERDA, you can save an average of 3% of your energy costs for every degree you turn up the thermostat. Similarly, Mass Save suggests raising the thermostat by 8 to 10 degrees when you leave the house, and it specifically recommends 78 degrees Fahrenheit as a good standard temperature. Likewise, the US Department of Energy advises setting the temperature “as high as is comfortably possible in the summer” to maximize energy savings.

These energy-saving tips work whether you have a window unit, a portable AC setup, or a central air system, like a heat pump. If you have a smart thermostat, you can even automate it so the thermostat automatically turns up (but not off) when you leave the house and then automatically sets to 78 degrees Fahrenheit when you return. Most of our window and portable picks also have smart-home features that will let you adjust the temperature even when you’re not home.

There are still plenty of situations where it might make sense to turn the AC off during the day. For example, some municipalities offer Demand Response programs or have fluctuating hourly electrical prices that vary based on demand. Of course, if everyone left their AC running all day, it could still increase the overall demand on the electrical grid (though inverter AC units can mitigate that strain).

But, generally speaking, if you want to get the best cost and comfort, set the temperature on your AC unit to 77 or 78 degrees Fahrenheit, and just leave it running. Bonus points if your unit has an “auto” setting, and if you’ve made sure the place is weather-tight and insulated. It might not bring back the summer of ska, but it will save you some money.

This article was edited by Harry Sawyers and Maxine Builder.

Thom Dunn

I’m a writer with a focus on heating, cooling, and humidification devices, as well as gadgets such as Christmas lights and karaoke machines.

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