> My water heater is not a storage device for electricy.

Criminy. Of course it isn't. It's a storage device for HOT WATER.

I have an experiment you can try. Unplug your hot water heater. See how long the hot water lasts. (For me, it remains hot enough to shower for TWO DAYS.) That suggests, to ignorant me, that one does not need to heat the water in it at a moment's notice, but it can be deferred to the cheaper times of the day.

Furthermore, water doesn't have to be heated to a precise temperature. The temp of your shower is controlled by mixing it with cold water. Hence, one can heat the water to a much higher temp when electricity is cheap, further extending the "battery" effect of storing hot water.

If you know ahead of time that you are going to use it like this, you can use a purpose built Thermal Store. It's a huge insulated water tank that you heat up, and then you extract heat from it by running mains water through a heat exchanger with the tank water. Typically used by people with wind, solar, or burned fuel energy supplies to smooth out energy consumption and soak up extra energy when there is too much. Also by people with variable electricity pricing to soak up free excess energy from the grid at night and heat their home for free all day.

Your water heater quite probably is an electrical storage device, it's been a fairly standard tech for decades in many countries, often using cheap coal or nuclear power at night since they aren't economic to ramp down.

I'd be very surprised if France didn't do things like this. edit: googled it:

https://thegoodlifefrance.com/french-hot-water-tanks-tarrifs...

So, one of the "unacceptable" solutions to fighting climate change, is something that countries with the highest nuclear mix have been doing for ages.

I'm quite sure my water heater does not store electricity. It stores hot water, and it completely lacks the ability to turn hot water back into electricity. At best I can use electricity at surplus times to heat water more cheaply, but that doesn't make it 'an electrical storage device'. I cannot attach a light bulb to it, for example, so you cannot use it during dark, quiet days to compensate for the lack of renewable power.

I am amazed that anyone would infer from my post that I was suggesting using your hot water heater to generate electricity.

Probably your first line is what caused the confusion.

>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.

> unless you plan to retrofit a bunch of houses

I said it was a next step. Besides, the government building code constantly adds new requirements for energy efficiency. That's why we have double pane windows today. It was mandated for new construction.

> Otherwise broken pipes at best and deaths at the worst

??? It's a pile of rocks in a box.

If you are relying on your box of heated rocks to keep your house warm and cooling occurs faster than planned you are left without heat.

> occurs faster than planned

Then you didn't do the engineering properly.