I remember Colin Fletcher, years ago, writing in The Complete Walker about trimming the borders off his paper maps to save weight, which seemed like an insane over-optimization to me. But then, I'm not an ultralight hiker.
I am impressed folks are getting their loads down to 10 pounds though.
I'm not even remotely an ultralight backpacker, but I do count ounces (no matter what your weight limit is, you can't escape making tradeoffs to stay within it). Your hiking load is a great example of how quickly apparently insignificant quantities can add up. Saving fractions of an ounce multiple times gets you large savings far more quickly than you'd think.
For a solar panel to be useful you need:
- At least a few days without access to electricity, otherwise even at max power, you won't get as much charge as a similarly sized power bank
- Good sunlight, preferably in the summer (more daylight)
- No shade, which is the opposite of what you want in hot and sunny summer days
- Correct placement for your solar panel, for example, having it hanging from your backpack will only work if you have the sun in your back
- A large enough solar panel, these tiny panels you sometimes find on power banks are useless
- Compatible devices. Solar panels have a variable power output, not all devices support it, some of them just shut down charging. Your best bet is to use a compatible power bank, but that information is not often specified. Test it beforehand!
My experience with a solar panel is from two week-long music festivals in the summer, which would be almost ideal conditions. My experience was that over the course of a week, I got about the charge equivalent of a 10Ah battery from my solar panel (rated 10W, 300g), so about half the efficiency of that gummy bear battery, for the reasons cited earlier. Maybe I could have done better with a better panel and better planning, but I'd rather have a battery, much more convenient, and cheaper too. I want to enjoy the festival, not babysit my solar panel.
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.
Also, I am talking about these portable <1kg solar panels. The large solar panels that go in your car/van are another story.
I've had trips where solar would have mostly failed - 11 days of nonstop rain on the Continental divide trail in Canada, to be specific - but solar has worked for me really well in CA, UT, WY, CO, etc. the places where solar would have failed were pretty obvious in advance, too.
And it doesn't take much direct sun on a 15 or 20W panel to keep two phones and a steripen charged if you're not being crazy with the use.
A 20 Ah (77 Wh) power bank weight about the same as a 15W solar panel. That about 3 full (0-100%) charges on a typical smartphone. I think that would have kept your wife phone up the whole trip no problem, and no need to worry about the sun.
On a 11 day trek in the sun, yes, by all means take a solar panel. However, most people I know who do such long hikes usually have access to electricity at some point. But if it is not your case, well, you are the reason why these solar panels exist ;)