There is a report in The New York Times about a rumor that Apple is researching the use of solar power for charging devices. Apple is reportedly testing out solar and wireless charging technologies for the rumored “iWatch” device. The wireless charging would use magnetic induction. Apple is also rumored to be integrating a solar-charging layer in the screen.
There is also a possibility that there will charging the battery through movement, a method that that is used in many modern watches.
When a person swings their arm, it may be able to push power to the device when walking. Apple filed a patent for this technology in 2009.
[Source: NY Times]