Most programmable thermostats have batteries, but they aren't always required. The batteries in a programmable thermostat provide a little extra power that some of these units need to run big screens and bright backlights, and they also serve as backup power so that you don't lose your settings during a power outage. In some cases, you can remove a battery-powered programmable thermostat from the wall, program it in a more convenient location, and then place it back on the wall when you're done.