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 (a now discontinued 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.
