I suppose if it is used directly as heat, it is more efficient(?) than having to charge a battery to power a heat pump (aircon). But this heat storage is a sin

I suppose if it is used directly as heat, it is more efficient(?) than having to charge a battery to power a heat pump (aircon). But this heat storage is a single purpose device, where as a battery is multi-purpose (you could also run a washer, dryer, water heater etc). So you have to count the energy loss of a battery to heat, and compare it with the extra energy required to _also_ power those other devices. It's quite a hard question to answer...