Average electric power generation in the US, on the grid, is a bit less than 500 GW. So, 15 TWh is about 30 hours.

There are lots and lots of other salt formations, both in the US and in the world. Also, aquifers and exhausted oil and gas fields can be used to store hydrogen.

Those cave storage devices don’t exist yet.

Sure they do, but mostly for natural gas. The capacity in the US and around the world is enormous.

Neither do safe and cheap (to build) nuclear plants.