Turning the air conditioner completely off can save energy while a home is empty, but in very hot or humid weather it can also let indoor temperature and humidity rise too far. For shorter absences, a higher setpoint often works better than maintaining the occupied temperature or shutting cooling off without limits.
For a few hours, raise the setpoint instead of cooling an empty home
A programmable or smart thermostat can allow a moderate temperature increase and return to comfort before you arrive. There is no need to hold the same occupied setpoint while nobody is home.
In humid climates, do not ignore moisture control
If shutting cooling off leaves the home at high humidity for hours or days, odor, condensation, and mold risk can increase. Use a temperature or humidity limit appropriate for the home and equipment rather than a universal rule.
Pets, medications, and sensitive materials change the decision
A home empty of people may still have temperature-sensitive needs. Maintain safe conditions for pets and heat-sensitive items, and follow any relevant rental, insurance, or manufacturer requirements.
For longer trips, prepare both the home and the system
Clean filters when due, make sure accessible drains are in good condition, and use an away setting that limits extremes. If the system is old or has condensation problems, service before a long trip can reduce the chance of an unnoticed failure.
Sources
- U.S. Department of Energy — Energy Saver — Practical information on home heating, cooling, efficiency, and energy maintenance.