Originally Posted by thoer
Why the need for heavier wire with the solar as opposed to the alternator?
This question of wire size comes down to basic electrical theory. Larger wires will loose less voltage over a long distance than smaller wires. I have seen that with my 2013 Ford F-150 and my Escape 19 running down the highway all day that the dual 6 volt batteries never fully charge. ( according to the specific gravity of the batteries)
This is at least partially accounted for by losses created by the undersized "charge" conductor on the truck. Bigger wire might help here as well because the batteries will not charge correctly if the correct voltage is not applied. By the way the on board charger in the Escape never fully charges the batteries either, which I think is also related to low voltage.
I think the best solar info on the web is Handy Bob, even if you disagree with some of his ideas. HandyBob's Blog « Making off grid RV electrical systems work
He's a full timer in a big rig that never uses a generator.