Back of the napkin: hundred million iPhones recharging their 13 Wh battery once a day every day from zero to full for a year is about 60 % the electric energy that this power plant [1] makes in a year.

Not huge amount of energy, but not negligible either.

[1]: https://en.wikipedia.org/wiki/Agua_Caliente_Solar_Project

edit: I see someone else did the math as well.