I'd think that you should plan it out pretty early in the process since you're going to want access to the interior of the roof for mounting hardware, wiring ingress, etc.
So from a step by step point of view, would you get solar before a ceiling (ceiling will be wood, probably tongue and grove), or does that not matter?
I agree with Dan and Ben. I'd plan out all of the electrical wiring before putting in any insulation or building walls/ceiling.
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Professionally converting vehicles and making custom furniture with a combined 14 years of mechanical/electrical engineering experience.