> 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.

Can you point at a residential scale rock heat storage system that is available? I saw one startup in the UK trying this several years ago, but can't find them now, and haven't seen anything since.

> Heap pump water heaters work, but you need a large one to hold a days worth of hot water

Mine (for a household of 4) is 45 gallons - small by US standards - but we keep it at a high temperature (along with a mixing valve) to get higher "virtual" hot water capacity during peak usage hours. It serves us fine for daily usage, but we also have efficient flow taps and showers throughout the house so that helps too.

> If you want to store more water than a days worth of use then you need more storage

I don't think it makes sense to store hot water for days if the goal is to minimize energy costs. The standby losses would be pretty significant, even with a very well insulated tank.

If the goal is resilience to a grid outage, it makes more sense to have a solar PV array + batteries that you can use to run the heat pump water heater, or have a backup gas-powered heater.