Cool, I've set up my new big solar panel to test the output on (looks up) the first heavily overcast day we've had for a week.
Sun came out, measured 55 watts off it already, so theoretically half its output being delivered with some indirect morning sun. Looks like a winner! Will measure again when the sun is right overhead.
Just got about 90W during a break in the clouds so fuck it, I'm happy that it's capable of delivering what it was advertised as, so let's move on!
I really need to get a roof rack cage for stuff next, might have to put some timber slats in there in just the right places to frame the panel while driving so I'm not constantly fretting about something putting pressure down on it.
A packing system is going to be essential for this to work.
On the bright side, even in shade like this on a murky day it's putting out about 5 watts, so it's looking pretty promising for when the sun actually hits it.