What about loss of material due to evaporation? Sand batteries can retain their mass in an unsealed container, vs water batteries which would lose mass in an open container or be under dangerously high pressures in a sealed container.

Should you at the very least not have the energy difference between something like 20°C and whatever the max is (100°C for water, as you put). All the energy down to 0K will not be possible to extract. This should favour sand even more.

It is an interesting these technologies you compare. Yes, a sand battery is in potential capable of storing higher temperatures if the source can generate these temperatures. We also have to look at the heat transfer that will seperate both energy buffers if seen from an application point of view. The heat transfer in sand is very low and this intrinsic insulation of sand begins to be very interesting when larger volumes are used. Water has a problem that it needs an extra insulation layer and larger volumes would be less interesting in comparison. However water is faster in exchange and is interesting as smaller buffer with shorter bursts and intake of heat.

I have superficially researched both options (with the conclusion that I cannot use either, since my installation would be too small, and would suffer from severe heat loss due to an unfavourable volume-to-surface ratio - it makes sense to design thermal stores for a city or neighbourhood, not a household).

I'd add a few notes:

1. A thermal store using silicate sand is not limited by the melting point of the sand, but the structural strength of the materials holding the sand. You can count on stainless steel up to approximately 600 C, more if you design with reserve strength and good understanding of thermal expansion/contraction. Definitely don't count on anything above 1000 C or forget the word "cheap". I have read about some folks designing a super-hot thermal store, but they plan to heat graphite (self-supporting solid material) in an inert gas environment.

2. Heat loss intensifies with higher temperatures, and the primary type of heat loss becomes radiative loss. Basically, stuff starts glowing. For example, the thermal conductivity of stone wool can be 0.04 W / mK at 10 C, and 0.18 W / mK at 600 C.

3. Water can be kept liquid beyond 100 C. The most recent thermal stores in Finland are about 100 meters below surface, where the pressure of the liquid column allows heating water to 140 C.

4. However, any plan of co-generation (making some electricity while extracting the stored heat) requires solid materials and high temperatures.

Water is nice if you want to move the heat - steam makes it easy to pipe it somewhere else. Also you can get extra energy out of the phase change. If you are just storing heat and using heat exchange, I think bricks or rocks would work better than sand. But it really depends what temperatures you want to use the heat.

https://www.ted.com/talks/john_o_donnell_can_a_simple_brick_be_the_next_great_battery

How are you getting your sand that hot?