When we out of the home or office, and not in the car, our mobile or MP3 is power off, it will be depressed, but if we carry a solar backpack, we would not worry about it. we could charge the mobile or MP3 from the solar panel, it is very conveneince for us to get the power and it is free. There is a solar backpack.

The Bag Specifications

Detachable Charging Section: Solar charging section zips off and can be attached to another bag, bike rack, etc. with the included straps and clips
Volume: 1500 cubic inches (25 liters)
Dimensions: 18″ high x 12″wide x 8″ deep (46cm high x 30cm
wide x 21cm deep)
Weight: 4.5 lbs (2050 grams) including battery and solar panels Padded laptop pocket: Dedicated pocket fits up to a 17″ laptop (17″ MacBook, larger 17″ laptops may not fit
Wire channels: placed throughout the bag for cables and bladder tubes
Phone pouch: Sits on the shoulder strap and allows you to
charge with easy phone access
Removable waist strap
Built for Comfort: High density padding in the shoulder straps and
back. Mesh backing material for better air flow
Fabric: 600D shell made from recycled PET (soda bottles),which is
waterproof, lightweight and UV resistant

A good design solar backpack!