I’m extremely skeptical that a solar panel on a backpack can work well–the panel is unlikely to remain aligned very well, and there isn’t enough spare capacity to provide useful output with partial sun. Reports I’ve heard from people who have tried solar charging on the go bear out the theory. What can work is a static deployment in a base camp, but you don’t want that stuck on a backpack.
As for touring, I’d much rather have the weight on the frame hour after hour than on my back. And there are usually resupply opportunities on a road tour–keeping 3 liters on your back all the time just seems like masochistic overkill.
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