Hint: pumped hydro is, exactly, a gravity battery. A gravity "flow battery", if you like. It is #1, just now, because a lot already existed. They cost a great

Hint: pumped hydro is, exactly, a gravity battery. A gravity "flow battery", if you like. It is #1, just now, because a lot already existed. They cost a great deal to build, back when. New ones would cost as much, but a small variation is much cheaper, not needing a dam or a watershed. A small variation on a bad idea can be a very good idea.