Is it better to run the AC all day or just at night?

Whether it is better to run the AC all day or just at night depends on climate, humidity, home construction, and energy costs, but for most homes in hot and humid climates, it is better to run the AC all day at a consistent setpoint rather than turning it off during the day and only running at night. Running the AC all day maintains stable temperature and more importantly controls humidity, preventing mold growth, protecting wood furniture, electronics, and drywall, and keeping indoor air filtered. Modern inverter and variable-speed air conditioners are designed to run continuously, modulating capacity to maintain setpoint efficiently, and they use less energy maintaining a steady temperature than cooling down a very hot house that has baked all day. If you turn off AC all day, your house can reach 85 to 95 degrees and humidity can rise above 70 percent, and then the AC must work much harder for several hours to pull down temperature and humidity at night, using more energy and causing more wear. The most efficient strategy is to use a programmable thermostat to set temperature 7 to 10 degrees higher when away, for example 78 to 80 when away and 74 to 76 when home, rather than turning off completely. In dry climates where nights are cool, you can save by using windows and fans at night and AC only during peak heat, but in humid climates like Ontario, Texas, or Florida, continuous operation with setback is more efficient, comfortable, and healthier for the house.