Latest in my long search for the perfect road trip backup battery is the Opteka BP-SC6000. This $20 6000mAh USB charging battery sports a solar panel!
Due to the various charging challenges presented by my 1970s vintage motorcycle I do not like plugging things into it, so I've used a number of pocket sized backup batteries to keep my phone going on the road. You can get a lot more juice for around the same money from a number of batteries Mark, Xeni and I have reviewed, but this one has a solar panel. I wanted to see how it'd do charging from my iPhone holder on the handlebars.
Charging is slow going via the solar panel, but it does work. Waze, the GPS mapping app I use on my phone, eats a lot of power. As the phone was relegated to a pocke, giving only spoken directions, while the battery had the handlebar perch, its screen was off and I am sure that helped. I actually don't put the iPhone in the perch very often, so this was fine by me. The results? In 6 hours of riding, much of it in direct sun, this battery was able to keep my iPhone 6, screen on, at 100% charge and itself at around 75%.
On a single day ride I'm not sure the solar panel makes a difference in total capacity from the 10k mAh battery I usually carry. When I am camping, and not stopping someplace with an outlet each night to recharge, this battery will definitely come with.