Perhaps surprisingly, the round-trip efficiency of an energy storage medium becomes relatively unimportant when the top-line cost of production gets low enough. Other costs come to dominate.

The odds-on favorite for utility-scale battery storage is iron-air batteries. Nobody even mentions the round-trip efficiency of those, so it must be pretty awful. But they are darned cheap. We can expect solar, in particular, to be routinely massively over-provisioned, so that much of the time most of the output would otherwise be wasted if not used opportunistically to charge batteries or hydrolyze water to synthesize H2 or NH3.