While literally zero loss is a violation of thermodynamics, with a really large tank and good insulation, it gets to self-discharge rates lower than most batter

While literally zero loss is a violation of thermodynamics, with a really large tank and good insulation, it gets to self-discharge rates lower than most battery chemistries. A few percent per month. There's a small district heating system in Alberta driven with solar thermal power. It uses a large insulated soil heatsink. Most of the energy input is in the summer and it took four years to fully charge up to operating temperature. It holds enough heat to provide all required heating through the winter for ~50 large houses.