To calculate the watt hours held in a sand battery, we need additional information about the specific heat capacity of the sand and the duration for which it is held at the given temperature. Unfortunately, sand is not commonly used as a material for energy storage in batteries, so it's challenging to provide an accurate estimate without specific data.
If you could provide the specific heat capacity of the sand (in J/(kg·°C)) and the duration (in hours) for which it remains heated, I would be able to assist you in calculating the watt hours held in the sand battery.