Yeah, I've learned that whatever they tell you, double it....
I've the feeling that 'Solar' belongs in the 'Black Arts' category..
No person likes the previous set up... and they have their own idea of what YOU need and how to set it up...again, double it... especially the wiring gauge...
You need to figure out what you need and future use possibilities... or you end up rewiring, moving things around or running outta room...
So, while the good ones, installers etc, might be and probably are expensive, they're more likely to set it up with something you can use...
Good luck, may the "Force be with you..." hehehhh, fun in the Sun as they say..