Abstract: A computer-controlled, uninterruptable power supply system is disclosed. The system selectively operates from more than one input source of power, e.g., an AC input, an external DC input and internal batteries. The system is of the type which utilizes one or more switch mode converters and is especially suited for use in computer systems. The system monitors a number of functions under computer control and reacts automatically to the monitored functions to follow a programmed course of action. During start up, the system monitors prescribed functions and prevents power turn-on upon detection of prescribed faults. The system may communicate with a host computer, provide data thereto, and receive down-loaded parameters therefrom. Such down-loaded parameters may be used by the system to control operation of the system.