The surface of the continental United States has an area of about 8 × 10 6 km 2 and an average...

The surface of the continental United States has an area of about 8 × 10⁶ km² and an average elevation of about 500 m (above sea level). The average yearly rainfall is 75 cm. The fraction of this rainwater that returns to the atmosphere by evaporation is   the rest eventually flows into the ocean. If the decrease in gravitational potential energy of the water-Earth system associated with that flow could be fully converted to electrical energy, what would be the average power? (The mass of 1 m³ of water is 1000 kg.)