To find the most efficient way to use our AC, we set up two identical units (the 12,000-BTU Midea U version, our choice) on two different floors of the same residential building. We also took measurements from the third apartment, on a separate floor, which was equipped with a heat pump (the now discontinued 18,000-BTU LG LAN180HYV3).
The heat pump and one window were in houses about the same size—about 1,200 square feet—with south-facing windows. Another Midea window entered the third apartment, on the top floor, and was about two-thirds that size, with windows facing west. Differences in height and direction of sunlight can obviously affect the test results, but I also thought it would be useful to compare the energy consumption of houses with different thermal envelopes under the same weather conditions. (Pro tip: Insulation and weathering make a big difference!)
On the first day of our test, we turned off the AC units at noon, and turned them back on at 5 p.m., after the sun had set high over the building. We recorded the temperature inside each apartment before we turned on the AC, then tracked how long it took to bring the indoor temperature back 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 a power monitor to track how much electricity each AC unit uses during each 24-hour period.
On both days, the outside temperature reached 93 degrees Fahrenheit.
To our surprise, all three apartments ended up using more energy when the AC was changed is turned off than when we leave it running all day. The first-floor unit, with its 18,000-BTU heat pump, used 7 kWh of energy the day we turned off the AC, and 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, a second-floor apartment, with a 12,000-BTU window AC unit, uses 18 kWh when we turn off the AC and 12 kWh when we keep it on—a price difference of about $1.27 per day.
The west-facing unit in the third-floor apartment—which was smaller than the other two apartments but also had a better-sized 12,000-BTU window AC unit—saw the least change in energy use: 12 kWh the first day the AC unit was turned off and 11 kWh when the unit was kept running. That may seem like a small difference, but it can still save you about 20¢ a day.
This experiment showed us not only how to save energy but also how to keep our homes comfortable. The day we turned off the AC units, the temperature inside each apartment rose into the low 80s; even after we turned on the AC units, it took another 90 minutes to cool the apartments again. In fact, only the apartment with the 18,000-BTU heat pump was able to get back down to 72 degrees Fahrenheit before sunset. In contrast, the temperature in the two rooms with window AC units hovered around 75 degrees until a few hours after dark, when the ambient temperature outside also dipped.
If you have to wait that long to cool your home after turning off the AC, you also don’t have to deal with the high utility bills that come with it. Of course, you may be able to schedule the unit to turn on with enough time to cool your home before your return. However, you will have to do a lot of work, and it will still cost you more money than if you had kept the AC running in the first place.