An example scenario where you would need an additional charge controller is if your solar array is larger than what your charge controller can handle. One way to determine this is to multiply your system voltage by the charge controller's amperage capacity. Let's say you have a 48V battery bank system and you purchase the MidNite Solar CLASSIC 150 that has a 96 amp maximum capacity. You would multiply 48 volts x 96 amps = 4608 watt capacity. If the combined wattage of all of your solar panels exceeds 4608 watts, you would need an additional charge controller.