A user-friendly, network-connected learning thermostat is described. The thermostat is made up of (1) a wall-mountable backplate that includes a low-power consuming microcontroller used for activities such as polling sensors and switching on and off the HVAC functions, and (2) separable head unit that includes a higher-power consuming microprocessor, color LCD backlit display, user input devices, and wireless communications modules. The thermostat also includes a rechargeable battery and power-stealing circuitry adapted to harvest power from HVAC triggering circuits. By maintaining the microprocessor in a “sleep” state often compared to the lower-power microcontroller, high-power consuming activities, such as learning computations, wireless network communications and interfacing with a user, can be temporarily performed by the microprocessor even though the activities use energy at a greater rate than is available from the power stealing circuitry.