An automatic thermostat which automatically reprograms itself after a power outage without the need of a backup battery or other storage device includes a temperature sensor, a set temperature potentiometer, a setback temperature potentiometer and an ambient light sensor. The thermostat also includes a microprocessor which is responsive to signals from the temperature sensor, the set and setback temperature potentiometers and the ambient light sensor. The microprocessor controls a display which displays the set temperature, the setback temperature and the actual temperature. A power outage detector detects when there is a power outage and triggers the microprocessor to reprogram itself by rereading the signals from set temperature potentiometer and the setback temperature potentiometer. A relay is controlled by the microprocessor to selectively provide power to a heating or air conditioning unit.