A system and method for controlling a vehicle-based source of uninterruptable power is disclosed. The vehicle-based UPS includes an energy storage system located on-board a vehicle and configured to generate DC power transferable to an external load, and an DC-AC inverter connected to the on-board energy storage system to receive the DC power therefrom and invert the DC power to an AC power useable by the external load. The vehicle-based UPS also includes a charging device located on-board the vehicle and connected to the on-board energy storage system to provide recharging power thereto and a control system. The control system is configured to determine one of a state-of-charge (SOC) and a voltage of the energy storage system and selectively operate the charging device to provide the recharging power to the energy storage system to maintain the SOC or voltage of the energy storage system within a pre-determined range.