Method for controlling an uninterrupted power supply system comprising multiple converters and a control device, whereby 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, wherein the step of determining a number of required active converters based on the system load level and a system efficiency depending on the number of active converters comprises determining the system efficiency depending on the number of active converters and a known converter efficiency depending on a converter load level, wherein the step of determining the system efficiency depending on the number of active converters and a known converter efficiency depending on a converter load level comprises the steps of determining based on the system load level and the number of active converters a system efficiency determining based on the system load level and the number of active converters a system efficiency when one converter is removed and/or when one converter is added, determining a highest system efficiency from the determined system efficiency based on the number of active converters as well as the number of active converters when one converter is added and/or when one converter is removed, and the step of determining a number of required active converters based on the system load level and a system efficiency depending on the number of active converters comprises selecting the number of active converters with the highest system efficiency, characterized in that the step of selecting the number of active converters with the highest system efficiency comprises comparing the highest system efficiency with a current system efficiency, and selecting the number of active converters with the highest system efficiency in case the highest system efficiency is superior to the current system efficiency together with a pre-defined threshold value, wherein the threshold value defines a minimum threshold which has to be exceeded in order to modify the configuration of the uninterrupted power supply system. 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, wherein the step of determining a number of required active converters based on the system load level and a system efficiency depending on the number of active converters comprises determining the system efficiency depending on the number of active converters and a known converter efficiency depending on a converter load level, wherein the step of determining the system efficiency depending on the number of active converters and a known converter efficiency depending on a converter load level comprises the steps of determining based on the system load level and the number of active converters a system efficiency determining based on the system load level and the number of active converters a system efficiency when one converter is removed and/or when one converter is added, determining a highest system efficiency from the determined system efficiency based on the number of active converters as well as the number of active converters when one converter is added and/or when one converter is removed, and the step of determining a number of required active converters based on the system load level and a system efficiency depending on the number of active converters comprises selecting the number of active converters with the highest system efficiency, characterized in that the step of selecting the number of active converters with the highest system efficiency comprises comparing the highest system efficiency with a current system efficiency, and selecting the number of active converters with the highest system efficiency in case the highest system efficiency is superior to the current system efficiency together with a pre-defined threshold value, wherein the threshold value defines a minimum threshold which has to be exceeded in order to modify the configuration of the uninterrupted power supply system.