When we speak of the efficiency of any machine, we are simply referring to how well it can convert one form of energy to another. If one unit of energy is supplied to a machine and its output, in the same units of measure, is one-half unit, its efficiency is 50 percent.
The overall efficiency of a centrifugal pump is simply the ratio of the water (output) power to the shaft (input) power and is illustrated by the equation below:
Ef = PW / PS
Pw= the water power
Ps= the shaft power