An electric power system includes a variable-frequency drive and a control module. The variable-frequency drive includes a first power converter, a second power converter and at least one energy storage module. The first power converter is connected between a DC bus and a power terminal. Moreover, electric energy is transferred between the first power converter and the power terminal at a first power. The second power converter is connected between the DC bus and an electric/kinetic energy conversion device. Moreover, electric energy is transferred between the second power converter and the electric/kinetic energy conversion device at a second power. As the second power is dynamically changed, the control module controls a charge/discharge operation of the at least one energy storage module. As a consequence, the operational change of the electric/kinetic energy conversion device is reversely compensated.