Please guys lets not argue about what you can buy and what not.
I know that you can buy ready made solutions (like this one I found shortly after I finished my charger:
http://www.amazon.com/Solar-Lighted-.../dp/B006E95V8M)
But I wanted it to be one single piece, a solar powered internet device (the first in the world!)
Now back to topic:
If you plan to charge your kindle directly via the usb port, you need a higher charge rate than you can supply with a manageable size solar cell. The built in charge controller (software) needs significantly more energy than the charge controller i am using (a couple of uA).
So if you want to charge your kindle via a solar usb solution you first have to charge an external battery (like the one built in in the device geekmaster showed us). Once this battery is full you can load the kindle battery (now that works because your charging current is higher)
But with such a construction your overal efficiency is about 1-2%
I tried once to load my cellphone (old Nokia) with a (bigger) solar cell directly attached to usb. It discharged my phone.
With that project I basicly wanted to show that it is possible. Perhaps some day amazon will sell solar powered kindles, wouldn't that be amazing?!