Earlier quoted context omitted.
Where are you getting 250 milliseconds and needing 15,000 from? Wikipedia claims US yearly electricity consumption is 4,090 TWh, and Bath County stores 24 TWh, with a generation capacity of 3,003 MW. That comes out as enough storage for a couple of days worth of US electricity use. Obviously you'd need 3 more to cover generation capacity.
Ah, I rechecked, I actually lost 2 orders of magnitude. It's 25 seconds, not 250 milliseconds. However, you lost 3 orders of magnitude: Bath County is 24 GWh, not TWh. My calculation was done under assumption of 101 quads of annual US energy use, but I'll redo this calculation with assumption more charitable to your argument. US uses only 4090 TWh worth of electricity, which is about 13 quads, but it also uses about…
Anyway. 100 of that scale might be better or easier as 1,000 smaller capacity, but even then we can reduce further. Some percentage of the grid is nuclear (roughly 20% of US electric) which would not need underwriting nor would 100% of continental wind power ever be becalmed. I would further assume that any concerted national effort to electrify carbon neutrally would come with a matching efficiency drive - insulation, tax inefficient users, take first steps toward a national grid (that TIL the US apparently does not have), etc to allow you to use the power you have rather better.
Yet ignoring all that, the US must have thousands or tens of thousands of disused mines and quarries, probably many open ones with an accidental lake in their place that could be repurposed. If there's adequate volume, you don't need much head of water. There's no shortage of hills and mountains with rainfall or snow melt that should give plenty of potential spots, population and environment permitting. The one thing America does not lack is space - of all terrain types.