This document provides an introduction and over view of state-of-the-art energy storage technology including a comparison of them and various evaluations of the new possibilities they give to the energy infrastructure and to society. It also describes which function each energy storage system is best suited for and which needs they can fulfill in our modern world. As a follow up, basic concepts about batteries are explained and a methodology to evaluate the capacity of most types of rechargeable batteries is described along with the design of a device required.