>The simplest energy storage solution is your hot water heater. Turning on everyone's hot water heater when there is surplus electricity, and turning them off when there's a deficit, is a very low cost solution.
People want a storage solution, not a surplus use solution. Using peak sunlight or wind to heat water isn't going to be useful for a factory or server farm. The only thing that sort of load shifting helps is to diminish peak load.
Yeah, it is probably a great idea to charge your electric car during the day at work as opposed to at night. Maybe a good idea for hot water too, until you have company or teenagers who manage to use up your water in the afternoon.
The pile of rocks to store heat sounds like a nonstarter to me, unless you plan to retrofit a bunch of houses and hope that the heat stored isn't depleted faster for any reason. Otherwise broken pipes at best and deaths at the worst. Not sure how much space you would need for something like that but seems like a problem for small apartments, either the heat leakage would overheat the place or you wouldn't store enough to keep warm.
