I've seen a lot of people asking if there will be a solar-powered charging option for their Storm Electric BIke and it makes me realize the general population doesn't realize what a PV panel is actually capable of.
Basically under ideal conditions you will get 1,000 watts per square meter (and really the commercially available panels aren't this efficient at all, they perform worse) of solar panel at the equator, at noon, with zero cloud cover. That means for the 350 watt hour battery to charge in an hour you'll need 542.501 square inches of PV panel to charge it in 1 hour's time... so you aren't going to just fold a small PV panel up in your backpack and set it out at your destination to charge the bike. Fore more information on solar power, see the bottom of this post for some good educational links.
HOWEVER if you are going camping or in the event of an extended power outage (keep in mind, solar panels only work if the sun is shining, storms can create cloud cover for days or weeks meaning no sun) want a way to charge the battery in a timely fashion to give you the ability to greatly increase your area of operation during the power outage, you CAN get a smaller panel which will charge the battery over several hours.
Something like the RenogyⓇ Foldable Solar Suitcase Battery Charger 100W for a good portable option
For the above you'd still need an inventer or charge controller and some cables to use it.
For a more permanent installation you could go with Renogy 100W Mono Starter Kit: 100W Solar Panel+20' Solar Cable+30A PWM Charge Controller+Z Bracket Mounts
You could also go with a larger capacity system for faster charging but you sacrifice portability,
Look at something like wind turbines (I like vertical turbines the best), if you want to read about wind-power I recommend two books Wind Energy Basics: A Guide to Home and Community Scale Wind-Energy Systems and Build Your Own Small Wind Power System. Things to keep in mind here, wind turbines will need a minimum steady wind to generate electricity, placement is crucial. Wind turbines also do make a bit of noise, in the form of a whirring or droning sound which can be quite annoying for larger turbines at speed but easy to ignore for smaller turbines at speed.
Yup, imagine a scenario like camping in an area with a lot of tree cover or power is out at home and no sun shining from a storm but you don't want to take your car out, maybe tress are down or debris is in the streets... guess what, you can use your AUTOMOBILE to charge that bike. How? The easiest way would be to buy an inverter to hook up to your car like the Cobra CPI 1575 1500 Watt 12 Volt DC to 120 Volt AC Power Inverter (authors note, I own this model and for it's price it's decent).
Further online reading:
For you DIY/hands-on types that want to learn about renewable energy by actually building something cheap check out this Instructable '9$ Solar, Wind and Hydro turbine (on your faucet) powered USB'.