Water falls from a \( 40 \, \text{m} \) high dam at the rate of \( 9 \times 10^4 \, \text{kg} \) per hour. Fifty percentage of gravitational potential energy can be converted into electrical energy. Using this hydro electric energy number of \( 100 \, \text{W} \) lamps, that can be lit, is: (Take \( g = 10 \, \text{m s}^{-2} \))