The present invention relates to an electric power control system and method therefor, and comprises a control unit 10 and a transformer 20 connected thereto. The control unit 10 detects varying input voltage in real-time, controls switching of triac devices equipped in the transformer, and applies control signal which switches triac device among a plurality of triac devices for target voltage output to the transformer 20. The transformer 20 outputs target voltage by switching tabs of triac devices according to control signal, supplies power by switching tabs of triac devices corresponding to target voltage included in control signal, and preventing EMF induced from primary coil to secondary coil by grounding the tabs of unused triac devices with N-phase for adjustment to the target voltage.