Heat storage has an aspect that was counterintuitive to me, but follows from basic geometry. It benefits greatly from large scale, since the ratio of the volume to surface area [1] decreases the larger you make a container. Accordingly, if a heat tank is large enough, the surface area becomes negligible relative to its volume, and it, in effect, becomes well-insulated by its own mass. For really big tanks, like might be used for an entire town to heat itself over a winter, practical self-discharge rates can be just a few % per month, which is better than most rechargeable battery technologies.
[1] https://commons.wikimedia.org/wiki/File:Comparison_of_surfac...