It's long been possible to add solar panels to your smartphone, ereader, or tablet, but it was only in the past few years that the tech has gotten good enough that a solar panel was a viable option for charging your mobile device, and not just an affectation.
Earlier this week I met with EnerPlex, makers of battery and solar panel tech. They showed me their Surfr iPhone 5 case, which they say can charge your iPhone in 14 hours.
The Surfr packs in a 2Ah battery which offers around 10 hours of talk time when fully charged, and it also has a solar panel on the back which can trickle charge your iPhone. I was told that 6 minutes of charging in bright sunlight is enough for around a minute of talk time.
I know that doesn't sound like much, but this is a significant improvement over what was available 5 years ago. At that time the available solar panel tech I was looking at buying was either much less efficient or much more expensive.
At that time it simply wasn't worth getting a solar panel to charge your ereader, but now it is. EnerPlex even has a line of solar panels built just to charge mobile devices in the back country.
Kickr launched earlier this year and is available from EnerPlex. The Kickr panels don't have an integrated battery, but you could conceivably use them to keep your tablet, smartphone, or ereader charged on a long hiking trip.
That is a good improvement over what was available only two years ago, when the Earl back-country tablet required a custom design. Now you can charge almost any mobile device and carry it with you.