Comment by apatheticonion

Yeah, sadly, it seems almost impossible to get anything higher than 30% efficiency (theoretically with a Stirling engine, if you can find one, haha) out of a thermal battery without extreme pressures and temperatures.

Back-of-the-napkin math felt promising. A 1kg block of sand heated to 500 degrees Celsius should contain about 100Wh of electricity. Scaling that capacity up is easy, as it's just about adding sand or temperature (+ an effective method of transporting heat across the sand - maybe sand + used motor oil?).

Assuming 80% efficiency, tariff arbitrage (buy electricity during off-peak hours and use it during high-price hours) would pay off very quickly. In my area (Australia) it would be a matter of months - but the low real-world efficiency and lack of parts make it impossible.

It could work for heating during winter, though perhaps an AC/heatpump with the condenser a couple metres underground would be better value for money.