A solar thermal power plant currently under construction will focus solar rays to heat a molten salt working fluid composed of sodium nitrate and potassium nitrate. The molten salt is stored at a temperature of 300 ^◦C, and heated in a power tower to 550 ^◦C. The salt has a specific heat capacity of roughly 1500 J/kgK in the temperature range of interest. The system can store 6000 metric tonnes of molten salt for power generation when no solar energy is available. How much energy is stored in this system?