A blistering stretch of summer heat has thousands of residential cooling systems working overtime, and with that extra demand comes a renewed debate over the best way to manage the thermostat. Some households still follow the old school advice to turn the air conditioner off when they leave and crank it back up on return; others maintain that letting a home warm up and then forcing the system to cool it back down wastes energy and shortens equipment life. The question matters to many because higher temperatures translate directly into bigger electric bills.
A few simple numbers help frame the decision. The U.S. Department of Energy calculates that allowing indoor temperatures to swing by seven to ten degrees for about eight hours a day can reduce heating and cooling costs by roughly 10 percent. In addition, Patrick Phelan, a mechanical engineering professor at Arizona State University, estimates that each degree a thermostat is raised can cut cooling costs by about 3 percent. Those figures indicate that deliberate, sustained thermostat setbacks — for example while the household is at work or on vacation — can produce measurable savings, but they do not automatically resolve how short absences or local climate conditions should be handled.
Wall-mounted Fujitsu thermostat showing 70°F — an example of the settings homeowners can adjust to trim A/C costs during extreme Phoenix heat.
Not every absence is the same, and not every homeowner needs to be constantly adjusting the dial. "If you’re gone for like 15 minutes to go to the grocery store, you don’t get any gain," said Elizabeth Hewitt, a professor and urban planning expert at Stony Brook University. By contrast, she said, if occupants are away for a typical workday of about eight hours, "you’ll almost always save more energy and money by turning things off." Hewitt’s distinction points to a practical rule of thumb: short trips do not justify altering a carefully balanced indoor setpoint, while longer periods away usually do.
Local humidity complicates that guidance. In arid regions, such as parts of Arizona, interior air can be allowed to warm up with relatively little consequence: dry air will not carry the same moisture load and can be cooled again without large additional dehumidification penalties. But in humid areas, a warmer home can also become damper. That extra moisture not only makes the space feel hotter, it increases the work the air conditioner must do later to remove humidity, and it can raise the risk of mold growth. Homeowners in humid climates therefore have to weigh the benefits of thermostat setbacks against those moisture-related drawbacks.
The question of equipment stress and repair also factors into the calculus. Phelan cautions that an air conditioning unit often requires 15 to 30 minutes after start-up before it is operating at peak efficiency. Repeatedly switching an A/C on and off for frequent absences could increase mechanical wear because the system must spend that initial run time repeatedly ramping up. That additional workload may not show up on the electric bill alone; it could lead to more frequent service calls to HVAC contractors over the long run.
One technological approach to balancing savings and convenience is the smart thermostat. Phelan estimates a smart thermostat can yield about 10 percent savings because the device learns household patterns and raises indoor temperature automatically when no one is present. The automation reduces the need for manual intervention several times a day and can coordinate setbacks around real occupancy schedules. In practice, that means a homeowner with a learning thermostat might program or allow the device to nudge settings up during work hours and restore comfort levels shortly before occupants return, eliminating much of the guesswork.
Other measures that complement thermostat management can produce additional relief without directly increasing A/C runtime. Opening windows at night to let in cooler outside air can be effective in dry climates; bringing that cool air in overnight can reduce the temperature A/C systems must counter during the following day. But in humid regions the benefits of night ventilation are limited, because that cooler night air can also carry moisture that the A/C later must remove. Simple alterations to how sunlight enters a home can also reduce cooling loads: closing blinds, especially those designed to reflect solar energy, can make several degrees of difference indoors. Window films that tint or otherwise cut solar gain are another option cited for reducing the amount of heat that reaches interior spaces.
Taken together, the practical guidance offered by energy officials and researchers points to a few consistent, evidence-based actions: use thermostat setbacks for multi-hour absences, consider climate-specific approaches to ventilation, and deploy technologies such as smart thermostats to manage setbacks automatically. Homeowners should also keep in mind the trade-offs between short-term electricity savings and potential long-term wear on equipment. With seasonal peaks expected to persist through heatwaves and into the hotter weeks ahead, households have time to evaluate which combination of settings, schedules and home improvements will balance comfort, cost and system longevity.
For Phoenix-area customers, utility APS recommends raising the thermostat a few degrees when away from home and, for those on time-of-use plans, pre-cooling by setting it lower before 4 p.m. then raising it during the 4-7 p.m. peak window to cut costs further. This demand-response tactic builds on general setback advice for the region's dry climate. (Source: APS newsroom)
Energy Star and the Department of Energy provide practical setpoint guidance many utilities endorse: they recommend about 78°F (26°C) when people are at home, around 82°F for sleeping, and allowing higher setpoints (about 85°F) while away to save energy.
ENERGY STAR’s program-level estimates for certified smart thermostats put average heating-and-cooling energy savings at roughly 8 percent, a somewhat more conservative figure than some academic or vendor estimates but useful as a baseline for homeowners evaluating a purchase.
Arizona Public Service discontinued its residential efficiency rebate program — including point-of-sale smart-thermostat rebates — effective January 1, 2026 following an Arizona Corporation Commission decision; the utility’s Cool Rewards demand-response program, by contrast, remains active and continues to offer enrollment and seasonal participation credits for qualifying smart thermostats.
Public-health guidance from federal agencies notes that older adults, infants, people with certain chronic medical conditions and those taking some medications are at higher risk during extreme heat, so these households should prioritize reliable cooling or access to cooling centers rather than raising thermostat setpoints excessively.
In persistently humid climates, HVAC contractors and building-science guidance recommend addressing moisture directly — for example with a properly sized whole-house dehumidifier or a variable-speed air handler — because central air systems are not always designed to remove latent (moisture) loads efficiently when homes are allowed to warm during setbacks.