In reply to rockwing:
Cheap solar power banks are usually just a power bank with a fancy cover.
If it's for one short trip, I'd just get a large enough power bank.
For longer trips a separate solar panel and bank is good.
I borrowed a 9W Goal Zero from my brother for a a couple of months living on the beach in Mallorca and that was fine for keeping the phone and a cheap single charge bank topped up on sunny days, but the problem is that when the panel voltage drops the phone or power bank will stop accepting charge.
I believe Goal Zero's own power banks are designed to allow trickle charge at low voltage, so as long as you charge the phone off thetbank instead of direct you get a lot more out of cloudy days/partial shade.
I noticed they have a cracking offer on the goal zero on go outdoors, but I have no idea if they have sorted their shit out yet so I'd be wary of online payment...