From what I gather from reading on it the sun charges the pack and the pack retains the charge until you plug up a device, then it will draw off power to charge up the device. That way you don't need to have sun to transfer the power. The others solar packs that I have seen the phone needs to be plugged up and exposed to the sun to charge.That is pretty sweet. I wouldnt mind having one. How long does it last? as long as the sun shines?