How many additional panels can be added to this system?

Yes, you can add as many panels as you want , You will eventually have to upgrade the controller for more Amps, as the charge controller included will only handle up to 120W of solar panel input, you already have 66W solar input with this kit. so you can double the number of these panels using the same controller. the battery recommended with this this kit is a 100Ah deep cycle battery. Please feel free to contact Nature Power Tech support at 1800-588-0590 for any more questions.