A power interface enables a low-power device to be powered from an alternating current (AC) wall receptacle or light socket with automatic backup battery charging. The power interface of an embodiment comprises a transformer module that receives an input signal. The device includes a battery module coupled to the transformer module, and the battery module includes battery charging circuitry coupled to a battery. The device comprises an output controller coupled to the transformer module and the battery module. The output controller includes detector circuitry that detects a state of the input signal. The output controller automatically controls coupling of one of a transformer module output and a battery module output to a device output according to the state of the input signal.