I realize I'm late to the party but why the assumption that the case is solar? According to the translation of the Chinese at Reddit the case has an extra battery that charges along with the internal battery.
Quote:
Sixth Image
Double battery design extends life to several months
All New - internal battery and charging cover create a double battery system, effectively extending the battery life; when charging the device, both batteries are charged; when not charging, the charging case can charge the internal battery, providing you with months-long sleep time. A newly designed Kindle sleep mode can extend battery life to the greatest extent.
|
https://www.reddit.com/r/kindle/comm...zon_cn/d1yhkui
I don't see a mention of solar, but admittedly I've only done a small amount of research.