In reply to tracyclimber:
You could be better served by just taking a beefy power pack, recharging it at your re-stocking stops (get one that has a quick charge feature and a matching quick charge wall charger), and of course being a bit frugal with your phone usage. You might save some weight and prime hiking time that way.
Most panels work best when stationary and directly facing the sun, a long mid-day stop... And if they get intermittently shaded (happens a lot on a backpack), they need time to "recover" or reset the device charging (the phone cuts off the charging circuitry, since it does not get enough current). In a worst case scenario (zig-zags, trees or intermittent clouds the whole day), you might not get much charge at all, as it will be just starting and stopping the circuitry all the time with the phone "buzzing" as it thinks it's being constantly plugged and unplugged.
Panels are really best just for a basecamp scenario, in my opinion. The small 1-panel 5-7W ones ("10W" is probably just marketing, sorry) would struggle to even charge a phone in anything but optimum conditions (phones need at least 2.5W to trickle charge), the 3-panel 21W ones or bigger are harder to position on a backpack and pretty heavy, and you'd probably still need to take a power pack with you anyway.
IMHO, solar charging on a backpack doesn't work that well in the real world, unless you are hiking in a flat desert with sun on your back...
Post edited at 17:43