There is a really cool Kickstarter project going on right now, and it is a solar case for your tablet. The case has a backup battery that charges from the solar panel on the back of the case. This then charges your tablet. It is called the Kudo Sol. The kickstarter also has other products, but the Kudo Sol is the most noteworthy, in my opinion. The Kudo Sol also has USB output, so you can also charge your phone from the case.
If Kickstarter's aren't allowed to be posted here, someone please let me know and I will take it down. Thanks