Problem:

                Calculate the flow rate needed to produce 1 kW of power(in l/sec). The vertical distance is 90m .
 Assume an efficiency of 80% by the power plant. the flow rate 1kg/sec.

Problem Explained:
The flow rate refers to the amount of water flow in liters/second .
The vertical distance of 90m means that the water falls from 90m high.
The efficiency of 80% means that only 80% of the possible energy stored in the water can be successfully converted.

How to solve this problem
use this

m=mass
g=gravitational pull on Earth
= 9.8m/sec^2
h=height
90m
This means that power is energy over time. This is equal to the potential energy, mgh/sec.