Better yet, this website pretty much explains it all AND warns against using "Romex" altogether and why it's better to use stranded wire.
But, since the Grape Solar rep. said for 2 of these panels there needs to be a 30amp charge controller, I'm assuming there needs to be wire that can handle 30amps and according to the AWG table on the first page this system needs 7awg
About solid vs stranded wire it says: Almost all wiring is done with stranded wire or cable. Solid wire is occasionally used for long runs, but in most cases it is not suitable for wiring panels, controls, pumps, batteries, or other components. If used, you run the risk of breaking the terminals....etc.
At the very bottom of the page, there's a link to go to their "Wire loss Table"....ok..well....since it's in metric and you have to cross reference, I'll have to come back to it. Anyone else decipher it?
On to a source for 7awg and specs on fuses, breakers and disconnects....yawn
Edit.....edit....edited to add a possible source for 7/2 AWG UF-B !!!!! But, is it stranded? Will call manana. My daughter sent me the link...she's purchased from them before. And how much is shipping to Hawaii...always a big question. Anyone wanna go halfers?
Edited.....It's specs say either solid or compressed stranded and the shipping is a whopping $114