The whole point of over-sized water tanks isn't to be the most efficient, only to supply a lot of demand when at times of the day when there is surplus energy.

The whole point of over-sized water tanks isn't to be the most efficient, only to supply a lot of demand when at times of the day when there is surplus energy. Right now, solar and wind are being curtailed more and more. Also, water consistency isn't that big a problem. Today's 120v heat pump water heaters store water at higher temperatures and using a mixing valve to deliver the desired temperature of water. I am just saying, surely electric water tanks, mixing valves, and temperature sensors are orders of magnitude cheaper than the amount of batteries needed to heat an equivilant amount of water.