How does the sun power falling water?

If by falling water you mean such things as rain, then the answer would be the Water Cycle. By that I mean the cycle by which the Sun heats up bodies of water causing them to evaporate, and also the Transpiration by plants sends water vapor into the atmosphere, there the water vapor collects and forms clouds and condenses to form rain. Then the rain falls into the bodies of water and get aborbed by plant life. and the cycle continues.

I hope this is what you meant by the sun powering falling water.