i'd do it like internet sales taxes : don't tax it until the tech is well established and market share is much greater.

first, there's a big start up R&D cost. add a tax and it becomes even less profitable. right now we are subsidizing it to give it a foothold, which i approve of. if we reach a day when most energy is renewable, then that's a time to look at getting rid of the subsidies and levying a tax.