Before I install my 20 amp controller, I want to make sure I understand Watts law correctly. I will be installing a 160 panel on the roof, so if Watts/voltage=amps, then 160/12=13.3, so a 20A controller should be sufficient? And if I wanted to add capacity to add an additional 80 W portable, 240/12=20, so I should still be okay? Or is there a factor I am missing?