Which is very little water. A couple of cubic metres. Storage of that
water would not be difficult. Obvuously a cube or sphere and all one
lump is best for losses. That can be a nuisance.

I'm not convinced it has no place - you would admittedly need a very
large additional thermal mass - a ton or two of water probably, to keep
the heating on overnight, with a 10-20C drop.

This is not of itself very expensive, though the places it can be easily
installed are of course going to be very limited.