A current detection module generates a first current detection signal of a first current flowing through a first power line and a second current detection signal of a second current flowing through a second power line. A multiplexer periodically switches between the two current detection signals in response to a control signal so as to output the first current detection signal within a first duration of each cycle of the control signal and the second current detection signal within a second duration of each cycle of the control signal. A signal processing module generates an energy accumulation signal according to the output of the multiplexer and a voltage detection signal of the voltage of a phase line. A control module provides the control signal to the multiplexer, and generates an effective value of the second current during the second duration.