Water has ~5x the heat capacity of brick. My first thought was a heat-pump water heater combined with radiant flooring. This way the heated water is stored in an insulated container until it is needed.
If insulation products can enable safe, economical storage at very high temperatures, sand can be used for energy storage, https://news.ycombinator.com/item?id=33696105
I wish the original article touched upon the electricity rates. If you can rely on off peak rates being less than 1/4 of the on peak rates, then I can see this making sense. Otherwise it seems better to use electricity more efficiently, exactly when needed.
Feolite
specific heat = 920.0 J·kg−1−1·°C−1,
density = 3,900 kg·m−3,
thermal conductivity = 2.1 W·m−1·°C−1.
maximum operating temperature 1000 °C.
Water
specific heat = 4184 J·kg−1·°C−1
density 1,000 kg·m−3,
thermal conductivity = 0.591 W·m−1·°C−1.
maximum operating temperature <100 °C unpressurised.That's an astonishing difference in capacity.
But then it can explode and destroy a small town.
Remember, hot water moved mountains.
I've been scoping out this company for some time, my boiler is currently in good shape but would love to switch down the road.
They heat up water via HeatPumps to required temp during the day when outdoor temps are higher and electric is cheaper. Max water temp can change based off excepted nighttime temp.
I really regret the choice, and wish I had gone with Artic with their integrated system for HVAC and water. It just makes so much more sense.
There are definitely not enough installers trained on heatpumps, and many just treat them as drop-in replacements for gas fired equipment, because that's what they know. It is rare to find someone who looks at the whole picture and also deals with the heat loss mitigation.
As it is a tankless water heater may be the easiest way to “fix” yours - or spend a big amount for a knowledgeable energy contractor.
I remember the website of one enterprising New Zealander, where he was using an under house water storage tank as a heat sink accessible by his heat pump!