Efficiency
Efficiency is a value used to express how much energy is added to a system, and how much of that energy is converted to useful work. Simply stated, it’s a comparison of what you put in, compared to what you get out. Efficiency is often represented by the Greek letter η (Eta).
Efficiency is a common calculation used throughout the engineering world for measuring waste. High efficiencies represent low waste whereas low efficiencies represent high waste. Waste may be in the form of energy, materials or time etc., all of which are often associated with financial loss. For this reason, high efficiencies are desired.