If a heater can retain the heat for a long time, then it can be set only to release heat when required, so a well insulated water tank can store the heat for a week, in the 70's some council flats were built with central heating, and I mean central, in the centre of the house was a storage unit that again would stay hot for a week, and the heat was removed with fans.
However insulation takes up space, so with non central heating where each storage radiator stores the heat independent, the only method to store enough in such a small space is to store at a high temperature, which means the minimum output is also high, at 900ºC losses are much higher than at 90ºC with a water system, but it takes up less space.
I was involved with trying to make a concrete bound brick rather than clay bound, it did not work out as expected as it was found the magnetite or iron ore, when heated to around 250ºC changes state, and releases load of oxygen which combines to hydrogen in the air to produce loads of water when first heated, once heated once it was OK, but that mean the bricks needed to go into an oven to heat them then allowed to cool before being fitted into the heaters, so may as well be bound with clay.
I had a heater in my office to test the bricks, and it did not go well, by 6 PM it was giving a very low output, which was not a problem for me, as about to go home, but in most homes that would be a huge problem, just as you want the heat, it has run out.
But my brother-in-law had the water system, main idea was solar panels, wood burner, and LPG gas could all produce the heat, giving him a multi-fuel central heating system, he could visit his daughter in Germany, and use the app on his phone on return to turn up the house heat, and his house was lovely and warm when he returned without using any mains electric just the stored energy in the tanks, on moving it was so good he looked at retro-fitting the system on his new house, however although OK with a new build installed when house was built, as a retro-fit the costs were far too high, looking at around £24k, which at 70 he would never get back.
So he just has standard I think oil, don't think he used LPG, like me his DNO fuse is 60 amp, so whole idea of electric only heating is a non starter, we have both looked at heat pumps, but they can't heat the circulating water to as hot as when we use oil or gas, so since water is cooler, the radiators need to be bigger, or fan assisted, using fan assisted the response time is fast. But again installation costs go up.
As to batteries, they would need to be huge, the unit is 23 kWh where normally we see batteries able to store 3 to 5 kWh, a ton of water can hold around 80 kWh so can be done with water tanks, but you need some where to store a ton of water, an ISO container used by many farmers is a ton of water. To use in the house we want the possibility of thermo syphon, so some where up stairs we need a reinforced floor which can take a ton weight.
The E7 tariff seems to be on its way out, so really today looking at heat pumps, I know my flat in Hong Kong had a heat pump, but that was used mainly for cooling although it could heat as well, but not cheap.
www.electricpoint.com
The description says output 1500w. However, the input says 3400w
