Abstract: A power management system is disclosed for providing an indication of the required available capacity (RAC) available from a backup battery unit (BBU) for backup or peak loading as required by a critical DC load, such as a computer bus. The power management system is configured to repeatedly determine the gross remaining capacity of the backup battery unit (BBU). The system processes this information and other measured or known battery parameters to determine the required available capacity (RAC). The RAC is based upon the required capacity of the critical load to which the BBU is connected. In general, the RAC is the difference between the gross remaining capacity of the battery at a given point in time and the required capacity of the critical load. In accordance with an important feature of the power management system, an indication of the RAC is provided.