An uninterruptible power supply system has a regulated power source, an electrical generator electrically interconnected to the regulated power source, an engine having a main shaft integral with or coupled to the shaft of the electrical generator, a rotating rectifier mounted onto a shaft of the electrical generator, a mains power supply, a switch connected between the electrical generator, the mains power supply and a synchronous machine of the regulated power source, a synchronous generator, and an uninterruptible load. The regulated power source has a housing, a synchronous machine, a synchronous generator, and a flywheel sharing a common shaft. The electrical generator has a shaft coupled to a combustion engine. The switch transfers power from the synchronous machine to the electrical generator to cause the electrical generator to rotate its shaft or the main shaft of the engine. A short-stop load is supplied power from the electrical generator.