> this device can gain 1 hour of charge in 20 minutes in direct sunlight, it would take on the order of 100 hours to collect the same amount of energy indoors

> this device can gain 1 hour of charge in 20 minutes in direct sunlight, it would take on the order of 100 hours to collect the same amount of energy indoors Depending how frequent and heavy of a user you are, the economics may work out such that they'll be passively charged enough and never require active charging. Related idea: It'd be cool if cellphones could charge in light whilst powered off. Then when they get old, they'll sit around and always be topped up and ready to go.