The following projects provide lots of information on portable battery chargers for USB, solar panels, and measuring the results of circuits to process raw power from the solar panel.

http://learn.adafruit.com/minty-boost/process

http://learn.adafruit.com/solar-charging-handbag

http://learn.adafruit.com/portable-solar-charging-tracker

And this wikipedia article explains the concept of boost convertors that generate a higher output voltage than the input.

http://en.wikipedia.org/wiki/Boost_converter

I’ve not yet had a go at creating any of these devices yet, at the moment it’s food-for-thought 🙂