Unfortunately it takes very little juice to recharge a cell phone. It probably takes far more energy and wattage to manufacture that thing than all of the electricity it will ever generate to recharge the phones.

I'm always dubious about just how eco-friendly such devices can possibly be.

First you have to make them... consuming considerable resources & energy and no-doubt generating waste.

And then you have the issue of the battery that has a limited life-span and is an eco-nightmare for disposal.