Setting a home thermostat a few degrees higher while away — rather than shutting the air conditioner off entirely or leaving it running at the usual temperature — is the best balance of energy savings, comfort, and humidity control for most U.S. households, three experts told the Associated Press. The recommendation applies broadly but varies by climate, building type, and the length of absence.
Air conditioning in U.S. residential and commercial buildings accounts for 4 to 7 percent of the nation’s total energy consumption, according to a study co-authored by one of the experts, making household cooling habits a meaningful lever on both utility bills and national energy demand.
Three engineering and urban planning experts told the Associated Press that setting a home thermostat a few degrees higher while away — rather than switching the air conditioner off entirely or leaving it blasting at full cooling — is generally the best strategy for most U.S. households during summer months. The guidance balances energy savings against humidity risks and equipment wear.
The U.S. Department of Energy states that adjusting a thermostat by 7 to 10 degrees Fahrenheit (4 to 6 degrees Celsius) for eight hours a day can save up to 10 percent a year on combined heating and cooling costs. Each single-degree Fahrenheit increase in thermostat setting yields about 3 percent in cooling cost savings, said Patrick Phelan, a mechanical engineering professor at Arizona State University.
How long you’re away shapes the math
For short trips, shutting off the unit entirely produces little benefit. “If you’re gone for like 15 minutes to go to the grocery store, you don’t get any gain” by turning off the AC, said Elizabeth Hewitt, a professor and urban planning expert at Stony Brook University.
For workday-length absences, the calculation shifts. “If you’re going for your work day, say for eight hours or so, you’ll almost always save more energy and money by turning things off,” Hewitt said. Even so, the experts said, raising the thermostat rather than switching the unit fully off can be preferable in many situations because of the strain from rapidly re-cooling a home that has warmed throughout the day.
AC systems can take 15 to 30 minutes after being switched on to reach peak operating efficiency, Phelan said. Repeatedly cycling a unit off and back on can also cause wear that leads to more frequent repairs over time.
Humid and dry climates call for different approaches
In dry regions such as Arizona, homeowners can raise the thermostat more aggressively and allow the home to warm more while they are away, the experts said. In humid regions such as Florida, turning the air conditioner off for extended periods can allow indoor air to become damp and harder to cool, and can increase the risk of mold since AC systems actively control indoor moisture.
The type of home also matters. Homes built with heavy materials such as concrete or brick hold cool air longer than older, draftier houses, said Gregor Henze, an architectural engineering professor at the University of Colorado Boulder. In less-insulated homes, Henze said, adjusting the thermostat even for absences of just a few hours can produce a measurable difference because indoor temperatures can rise quickly.
Unit type and smart thermostats
Window units are generally less efficient than central air systems because they are installed in an open window, making it difficult to fully seal out hot air, Hewitt said. She recommended inexpensive foam spray insulation as a low-cost fix, saying that “cheap foam spray insulation in open windows or areas that are drafty is a really low-hanging fruit that doesn’t cost a lot of money and really helps retain the indoor temperature in your home.”
Smart thermostats, which learn household occupancy patterns through built-in sensors and adjust the temperature automatically, eliminate the need to manually change settings throughout the day. “If you’re going from just an ordinary manual thermostat to installing a smart one like a Nest, then you can expect something like 10% savings,” Phelan said.
The national energy picture
Air conditioning for residential and commercial buildings in the United States consumes about 4 to 7 percent of the nation’s total energy, according to a study Phelan co-authored. “That is a lot. And that’s not counting, say, cooling going to data centers, which is a different category entirely,” he said.
Cooling without air conditioning
For households without AC or as a complement to it, all three experts pointed to low-cost passive strategies. Closing blinds can reduce indoor temperature by several degrees, Hewitt said. Some blinds are designed to reflect sunlight, and tinted window films are another option, Phelan noted.
Henze pointed to what he called “time-honored strategies” such as opening windows at night when outdoor temperatures drop. In dry climates, nighttime ventilation adds little moisture to indoor air. In humid regions, however, it can bring in dampness that the air conditioner will later need to remove.