A method for controlling an amount of power delivered by a power source (16) to a power line (32). An energy storage unit (38) receives portions of energy generated by the source and provides stored energy to supplement power output from the source to the power line. Maximum and minimum limits define a range of change in the power delivered to the power line by the power subsystem. An amount of power to be delivered by each of the source and the energy storage unit to the power line is based on (i) a determined amount of power available for delivery directly to the power line from the power source, (ii) a determined level of energy in the storage unit available for delivery to the power line, and (iii) the limits in the range of change in the power to be delivered by the power source to the power line.