The Voltaic backpack is a mobile power source, designed to charge your
gadgets without tying you to a power outlet.

Embedded in the back of the bag are three tough, light weight, waterproof solar panels, which generate up to 4 watts of power.

Inside the bag is a Li Ion battery pack. As in a solar powered house, the battery stores energy so it is available when you need it, not just when the sun is up.

(link)