I have a 3000L tank to do that. Just to give an idea of energy stored in there, I'll expand on the calculations done by

@solarsimon - maybe this gives you some ballpark figures to compare with.

The specific heat capacity for water is 4.2 kJ/(kg K). A 3000L tank, at 80C, stores in principle ~300kWh of energy, but let's assume the ambient temperature is 20C and that's what we start out with as the temperature in the tank. We want to figure out how much more we can store. Thus, the temperature delta in the tank we want to look at is 80°C - 20°C = 60°C

The calculation with all units:

*E = (4.2 kJ/kg°C) ((80 °C) - (20 °C)) (3000 liter) (1 kg/liter)*

= 4.2 * 60 * 3000

= 756000 kJ
Since 1 kJ = 1kWs we multiply this by 1/3600 (hours/seconds) to get kWh - this makes for around 210kWh.