[forgotpwagain]

You regenerate the water when H2 and O2 are recombined to produce electricity.

Dan Nocera ( http://nocera.harvard.edu ) is very prominent in the water splitting field and has done a calculation in which a volume of an olympic sized pool would need to be split every second to replace all power sources on earth ( https://poptech.org/blog/daniel_nocera_on_personalized_energ... ).

It seems like a lot, but it would presumably be decentralized across many many locations. Additionally, you don't need 24 * 60 * 60 * olympic pool volumes -- you can cycle the same amount of water into/out of a fuel cell in a closed system.

[maxerickson]

You are talking about several hundred billion liters.

The inland reservoir here is 100 billion liters:

https://www.google.com/maps/@43.8937611,-86.4337261,8900m/da...

It's not very much water.