[knc1]

The K3 (the 1750MaH battery) battery management includes a 500ma charge curve. About a 4 hour rate.

So anything up to a 5v / 2.5w rated solar panel would be useful.
Anything else of more capacity, the battery management wouldn't make any use of but maybe some of your other devices would.

From another view point, a 5v / 0.5w rated solar panel would closely match the battery management's 100ma charge curve.
Good for keeping your Kindle "topped up" when not using it.

Any of the Kindle models that offer a 3G option will have a similar capacity battery (even if they don't have 3G installed) since that is about the smallest capacity battery that will run the 3G transmitter.

Edit:
That probably explains GM's observation that in some Kindles, in some modes, the battery seems like it will never re-charge.
The battery management chip must default to the 100ma (20 hour) curve unless enough firmware is running to switch it to the higher curve rate.

Note: The above information is taken from the K3's built-in battery management (factory) diagnostics.