I've used this set up for two month-long rides and several shorter day or two trips so far. It's been enough to keep my Samsung Galaxy S5 smartphone and my 50-lumen rear blinky charged without ever needing a wall socket.
The CHOETECH 19 watt solar panel is a tri-fold design and was easy to tie across my rear rack. I chose it tested and reviewed good on youtube and elsewhere. It charges a 10000mAh brick battery 75 to 100 percent during a day's ride with good sun. (10,000 mAh will charge my phone about 3 times). I chose the Anker PowerCore battery because it claimed to be one of the lightest.
Looks like the price of the solar panel has gone up since I bought mine a couple years ago for around $40. Looks like it's 50 now. Still, I'd recommended it. Mine has seen a lot of use; just short of 3k miles on the bike -and it's as good as new as far as I can tell.
https://www.amazon.com/gp/product/B0...?ie=UTF8&psc=1
https://www.amazon.com/gp/product/B0...?ie=UTF8&psc=1