I'll also throw a vote for Anker. Reasonably priced, variety of sizes (physically and power-wise).
I have this one and it has held up to almost 2 years of usage. Small size, decent power reserves:
https://www.amazon.com/gp/product/B00P7N0320/ref=oh_aui_detailpage_o05_s00?ie=UTF8&psc=1
I also have an iMuto. I hear them talked about a lot less than Anker, and I bought this one on a "Lightning Deal" on a whim, though I don't recall the exact price, but I'd buy another one at 30 bucks if I needed one. It's bigger than the anker and bigger/heavier than a phone, but it fits in my pocket still and is more than up to the task of keeping a phone going all day:
https://www.amazon.com/gp/product/B01191S0VY/ref=oh_aui_detailpage_o01_s00?ie=UTF8&psc=1
EDIT: and the second charge port comes in really handy sometimes. I've been able to bail out other trainers at raids who need a charge but forgot their charger even when I'm charging, too. Also useful at community days if you don't mind being tethered to somebody else while walking around.
the d.light solar lanterns are wonderful. I have a couple of them and one of the task lights. For your needs I would be more inclined to buy couple of those, a $30 8 watt Anker solar panel or, planning ahead, in case you have other electronics, a $60 14 w panel. http://www.amazon.com/Anker%C2%AE-Dual-Port-Charger-PowerIQ-Technology/dp/B00E3OL5U8/ref=sr_1_1?s=electronics&ie=UTF8&qid=1424032021&sr=1-1&keywords=anker+solar
And of course, a $19 back up battery for cloudy, rainy days: http://www.amazon.com/5200mAh-Compact-Portable-External-Technology/dp/B00P7N0320/ref=sr_1_6?s=electronics&ie=UTF8&qid=1424032304&sr=1-6&keywords=anker+lipstick+charger