Electricity is used to heat up bricks. Bricks store heat for very long durations. Bricks transfer heat to other systems for industrial heating such as boiling chemicals or steam ironing clothes.
This replaces the fossil fuels used for industrial heat.
It's a heat storage system. Before water is used. Now we can use bricks.
[1] https://en.wikipedia.org/wiki/Coefficient_of_performance
Refer to it as COP or as moving 3 units of heat for one unit of input.
You can't kill what is already dead.
The right question is how much heat escapes? This will depend on time, and they seem to be claiming 12 hours effectiveness which would seem to imply half of the heat escapes at max storage somewhere between 3 and 24 hours.
It depends on both time and how full it is. Possibly also how full it has recently been as well in the case where it is filled unevenly to reduce loss.
Without anything more specific than '1% loss a day' from their marketing copy, it's difficult to say, but probably "about 90% in typical use" if you want to be even more reductive than that rather than encouraging clear communication.