> So I'd say you need at least a week without electricity in the best conditions to make a solar panel worth it, preferably more, which I believe is rather uncommon.

That's basically my use case. I have a "15 W" panel. I can get about 5 days from my iPhone for navigation, and most of my trips are 5 - 7 days, so really it's opportunistic charging for reading on my phone after dinner. I can generally get an hour or so of reading from just hanging the panel off the back of my pack, and another two hours from setting it in the sun during my ~1 hour lunch break if it's not so hot out that nothing charges. 300 g for ~3 hours of reading at night, indefinitely, is a good trade for me.