I'm pretty sure they can't turn the tap off when it comes to web indexing. And I also doubt the major cost for google is electric power for the machines doing the indexing - the power bill is probably negligible in the big picture of Googles business (if not I'll stort their stock)
It doesn't matter how the power bills compare to their overall bills, what mattes is how much power costs compare to hardware costs in terms of whether it makes sense to shift those things that don't have to be done in realtime.
