I had totally forgotten about the rain generator item in that edition of What If! I think because it includes this gem:
Q. What if you strapped C4 to a boomerang? Could this be an effective weapon, or would it be as stupid as it sounds?
A. it's the second one. Aerodynamics aside, I’m curious what tactical advantage you’re expecting to gain by having the high explosive fly back at you if it misses the target.
There's two kinds of boomerang, the one to try to hit something with has a different shape and goes in a straight line without returning. It indeed might be a clever alternative to a grenade, made of explosives in a thin metal shell because one person could throw more explosive mass further than a person could throw a grenade.
That's for a turbine setup. We need to do the math on a kind of rain powered flow battery instead. And it doesn't need to offset grid power, just handle cases where hookup is inconvenient and solar is off the table.
In short it won't be. Less than $100 in a century during the highest recorded rainfall in the US.