Indeed solar harvests much more energy than other (vibration, thermal) methods, assuming you are outside and not in an alleyway. Regarding the concerns raised in @BoRRoZ link, this applies to solar power also due to the switching regulator in the posted design. This can be avoided by using linear regulation and solar panels & capacitors of appropriate voltage rating. If you get the voltages right, the efficiency can even be better than switching regulation.
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