This paper presents a small-size electric power sensor that can be installed at an outlet to sense the power of electrical appliances whose plugs are inserted into the outlet. The data for the power consumption of the appliance and the power for the microcontroller unit (MCU) are obtained by an electromagnetic induction method, so that it can be implemented without any electrical work. The sensitivity of the power sensor is 1 W, and the power for the MCU can be generated from an appliance power of 23 W. The sensor itself consumes 3.75 mW. This sensor can be used to measure the power consumption of the existing electrical appliances.