That would require smart meters to actually be more than a dumb meter with a radio transmitter attached, to send in meter readings.  And there would be even more push back against them if people were told that their new smart meters could turn their appliances off and on at the whim of someone in a remote data centre.

I've often thought that as the grid itself is stabilized using frequency as a proxy for over/under-load, this is the easiest thing to sense in a smart appliance - no need for power hungry internet, no data privacy concern, just the device decides when to take itself off. The exact frequency threshold can be integrated with the urgency of the task and a record the last 24 hours worth of frequency readings, so if the water is cold, then whatever the frequency is, turn heating on, but if it is part warm then knock off early if the frequency is low and come back and top up when it recovers. It is also less 'bang-bang' control than telemeter switching of many MVA all at the same instant.

no profit or complex electronics needed, so not commercially attractive.
Mike

I have been thinking about this in connection with getting a PV array installed.

Heating 200 l of water from 10 deg C to 60 deg C takes 200 x 1000 x 50 x 4.2 = 42 MJ ≈ 12 kWh so this is an excellent way of storing energy in the spring/summer/autumn. Instead of using gas, we could use the immersion heater.

What this plan needs is a device which knows when to turn the immersion on and off so I shall be interested to hear more. I don't think that it needs to involve the smart meter, but it does need to know how much energy is available. It would save nothing to have 50:50 PV and grid.

Is something based upon a light meter available?

There are devices that monitor the household consumption and generated power and divert any excess power  to an immersion heater, e.g. https://solarimmersion.co.uk

I have no experience of this device but an internet search came up with several.

David

There are also devices that charge EV's the same way.

David

The grid just needs a way of ditching water heaters at peak times probably for an hour or 2 only on some winter evenings maybe a dozen or so times a year.

For solar heating of domestic water it could be more efficient to install vacuum tube or plate heaters on your roof as PV,s are only 20% efficient 

Well, yes, if the object was only to heat water.  This is a way to use excess electricity generated rather than feeding it into the grid for a pittance, use it to heat water for free that you would have to pay full price for at a peak time.