Brunton makes a range of flexible solar cells they call SolarRolls. These weather-proof solar cells can be rolled up quite tightly, and attached to a backpack. Seen here is a 14W model — it produces DC power even during low light conditions. SolarRolls come with a range of plugs, including a standard car lighter socket. They can also be attached together if you need more power. See a review and more photos of the SolarRoll at RainyDay Magazine. It’s available from Amazon for $399.