"The rate of heat transfer from the house to the outside, or vice versa, is dependent partly on the temperature difference between the house and outside. More heat is transferred when the difference is greater, so it takes more energy to keep the house at 72°F when it is 40°F outside than to heat the house back up to 72°F after you return. With air conditioning systems, the equipment runs at peak efficiency when it operates for long periods. Cooling the house back to the comfortable temperature will use less electricity than the unit would use cycling on and off for short periods to maintain the set temperature. If the house takes too long to get back to a comfortable temperature, you might investigate getting a programmable thermostat, and set it to start heating or cooling the house an hour or so before you return. You could also set the thermostat back, to a lower temperature for heating, or a higher one for cooling, while you are gone, rather than turning it off completely."