Water fall from a 40 m high dam at the rate of $9 \times 10^4 \mathrm{~kg}$ per hour. Fifty percentage of gravitational potential energy can be converted into electrical energy. Using this hydroelectric energy number of 100 W lamps, that can be lit, is: (Take $\left.\mathrm{g}=10 \mathrm{~ms}^{-2}\right)$