This is another crucial parameter defined as the ratio of electrical energy a battery supplies to the amount of electrical energy required to return it to the state before discharge.
A strong argument for using electric vehicles is based on their efficient use of energy, resulting in reduced overall emissions; hence high energy efficiency is desirable. From what has been said in the preceding sections, it should be clear that the energy efficiency will vary significantly with how a battery is used. For example, energy efficiency decreases considerably if the battery is charged and discharged rapidly. However, it does act as a guide for comparing batteries, in much the same way as fuel consumption does for cars.
It is adapted from the introduction of Electric Vehicle Technology Explained by James Larminie.