In an UPS system, following components are arranged in a machine cabinet: an AC-to-DC converter connected to a DC voltage link on its output side, a DC-to-AC converter connected to the DC voltage link on its input side; a motor/generator electrically connected to the DC voltage link and having a flywheel coupled to its rotor; a bypass to the AC-to-DC converter and the DC-to-AC converter connected in series, a bypass switch being arranged in the bypass; a controller; and a blower which draws air out of the surroundings through air filters, blows the air in the machine cabinet through a heat exchanger, and ejects the air again. The heat exchanger is arranged in a coolant circuit together with a coolant pump, cooling bodies for the power output stages, and coolant channels which include stator channels running through a stator of the motor/generator.