A power monitoring apparatus for receiving an external input signal includes a first modulating unit, a second modulating unit and a comparing unit. The first modulating unit receives and modulates an input signal into a first signal. The second modulating unit receives and modulates the input signal into a second signal. The comparing unit is electrically connected with the first modulating unit and the second modulating unit and has a first input terminal, a second input terminal and an output terminal. The first input terminal receives the first signal and the second input terminal receives the second signal. When a difference between the first signal and the second signal is higher or lower than a predetermined range, the output terminal outputs a control signal.