We have a 2017 18.58-2 with the J-Hub panel, ours is a J35C. Our 150W roof top panel runs at 18V, and our additional 200W folding external panel is the same voltage so it can piggy back into the same solar inputs (ie, into the built in regulator), however, if it was a different voltage, I would just go via the regulator on the back of my folding panel into the 12V Aux Power In connection. Voltages should be written on the back of your roof top panel if you wanted to confirm and match to another one.