> Sand/rock heat storage is for industrial and utility scale application, not your house.

I've seen them for a house as well. (when the utility offers a large discount for power used overnight - 40 years ago my local coop was trying to sell my parents on this, but since we had natural gas heat it wasn't worth it) To hold more than a days worth of heat you need a lot of rocks.

Heap pump water heaters work, but you need a large one to hold a days worth of hot water (40 years ago my family had 200 gallons of hot water storage - the utility gave my parents a discount on water heating because we only heated water at night, we ran out of hot water once that I remember). If you want to store more water than a days worth of use then you need more storage. This can become a drowning hazard if we are talking about a couple weeks worth.