A programmable or "set-back" thermostat saves energy by reducing the temperature difference between indoors and outdoors -- and thereby the rate of heat transfer to the outdoors -- while you're away from home or tucked in bed.  If you program it to bring the temperature back to a comfortable level before you get home or wake up, you may not even notice that it was changed!  Homes that are built of wood and drywall will save more energy with a programmable thermostat than homes that have a lot of concrete, tile, or stone indoors, because dense materials don't change temperature as rapidly as light ones.

We recommend programmable thermostats even for people who are home all the time and like to keep the temperature constant, because when they sell their homes, the next owners will probably not be home all the time.