A back-up power system having a monitoring system for determining, and for allowing remote monitoring of, a back-up time can be provided by a plurality of batteries for a given application, at all times. The back-up power system includes a plurality of batteries, each having an integrated circuit adapted to monitor individual battery's state of health. The back-up power system also includes a data management unit for evaluating the back-up time available from the plurality of batteries based on a sum of individual battery available capacity, a measured ambient temperature and a continuously updated measured application current load. The available back-up time and the measured application current load are accessible to a remote user via a communication link.