My point wasn't so much that solar on phones would be useless, as that there isn't enough energy in ambient indoor lighting to really charge much of anything. Most people don't appreciate the fact that the eye has a very nonlinear response to light: indoor lighting may appear about the same as sunlight to you, but in reality the sunlight is maybe 100 times as intense.
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Now if you're out camping, say, and leave your phone out in the sun, then solar would be useful. But how many people do that? Most keep their phone in a pocket or purse most of the time, and even if left on a table or desk indoors, the charging would be miniscule. Much more cost-effective to have separate solar chargers (which could charge/run any USB-powered device) for those who need such things.