The present invention provides a method for controlling an uninterrupted power supply system comprising multiple converters and a control device, the multiple converters are connected in parallel between a power supply side and a load side of the uninterrupted power supply system, each converter is provided to be individually activated by the control device to provide power from the power supply side to the load side, comprising the steps of determining a system load level at the load side of the uninterruptable power supply system, determining a number of required active converters based on the system load level and a system efficiency depending on the number of active converters, and activating the number of required converters based on the above determination.