Air conditioner on or off when leaving home: what saves more energy in summer?

Leaving the AC off for hours or days during peak summer can cause indoor humidity to rise. According to U.S.-based McGowan's Heating and Conditioning, trapped moisture can lead to mold, mildew, warped wood, and damage to books and paper goods, while also making the space feel hot and uncomfortable.

Once the system is turned back on, it has to work harder to cool the home down, which increases energy consumption. Frequently switching the unit off and on can also strain components like the compressor, reducing the system’s lifespan and leading to costly repairs, as reported by Austin American-Statesman newspaper.

Leaving air conditioner on at a higher setting while you are away helps prevent humidity build-up, and higher cooling cost. Photo from VnExpress/Linh An

"The answer boils down to how energy intensive it is to remove heat from your home," PBS reported. Factors such as insulation, the size and type of the air conditioner, and outdoor temperature and humidity all influence energy use.

Instead of shutting the system off completely, experts recommend raising the thermostat slightly while you are away, then lowering it when you return.

Bloomington Heating & Air suggests keeping the indoor temperature about 10–15 degrees Fahrenheit (5–8 degrees Celsius) cooler than the outdoor temperature. For example, if it is 90 F (32 C) outside, setting the AC to around 77 F (25 C) is considered ideal for both comfort and efficiency.

The New York State Energy Research and Development Authority says you can save about 3% on cooling costs for every degree you raise the thermostat. Similarly, energy provider Mass Save recommends increasing the setting by 8–10 F (4–6 C) when leaving home, with 78 F (26 C) cited as a good standard for summer.

The U.S. Department of Energy also advises setting the thermostat "as high as is comfortably possible in the summer". Smart thermostats can automate this process, raising the temperature when you leave and lowering it before you return, to help reduce energy use, the New York Times reported.