I've proposed this (water heater as battery) innumerable times here on HN. Glad to see it finally catching on!
You can do the same thing with the HVAC system. For example, when the sun is high and electricity is cheap, have the A/C cooling a large mass of stone or concrete. Then, blow air over it at night to keep the house cool.
The PassivHaus concept utilises a massive (5,000 gallon / 20k litre) insulated thermally stratified water tank as thermal storage mass, among other elements.
It's used to create zero-net-energy homes in Fairbanks, Alaska.
You are not the first to propose it, and won't be the last. It is obvious to any engineer who spends more than a few minutes thinking about the problem. It works in practice too, but the space needed means most people don't bother.
You can do the same thing with the HVAC system. For example, when the sun is high and electricity is cheap, have the A/C cooling a large mass of stone or concrete. Then, blow air over it at night to keep the house cool.