I'd like to make a solar powered USB charger, to charge my phone. I'd ideally like to use (a) solar panel(s) that I can 'mount' onto a canvas messenger bag, and use a pocket to hold my phone. Obviously I need to provide ~5V to charge a phone, therefore I need a solar panel that can provide at least that much voltage, which can thus be used with a 5V regulator to step the voltage down to the appropriate level.
What other criteria do I need a solar panel to meet? Is there a specific current that I need? Or at least a threshold current? What will the correlation be between the power output of the panel be and its ability to be used in this fashion? Just charging times?
In terms of my skills, I'm an Electrical Engineering major, but my knowledge is mostly academic, so I appreciate any real world advice.