At the dam, water falls from a height of 150 m on a turbine with an efficiency of 65%. This implies that 65% of the gravitational potential energy stored in the water is converted to electrical energy.

The gravitational potential energy of an object with mass m at a height h is equal to E = m*g*h. The mass of 3000 liters of water is 3000*1 = 3000 kg. At a height of 150 m, the potential energy is 3000*150*9.8 = 4410 kJ

The turbine can convert 65% of this to electrical energy. The electrical output is 2866500 J

When 3000 l of water falls from a height of 150 m a turbine with efficiency 65% produces 2866500 J of electrical energy.