A power conversion system includes an uninterruptible power apparatus, a generator module, and a control unit. The uninterruptible power apparatus includes a conversion module and a DC-to-AC conversion unit. The control unit controls the conversion module and the generator module according a power command so that a first average power provided from a DC power source coupled to the conversion module is slowly increased or decreased, and the control unit controls the conversion module according to a bus voltage so that a second average power provided from a mains is slowly decreased or increased corresponding to the first average power.