Since they also make stationary batteries I wonder what would it cost to manufacture small flywheel energy storage units for homes instead. To an extent, those

Since they also make stationary batteries I wonder what would it cost to manufacture small flywheel energy storage units for homes instead. To an extent, those are used in bigger grids to balance out peak production and peak consumption: they rotate in a vacuum on magnetic bearings and require very little if any maintenance. Those big boys probably cost a lot but a consumer-grade flywheel which could spin for, say, 24 hours and be slightly less efficient (wrt. input-to-output and friction losses) could still be a big deal to even out daily usage.