Storage is an important domain of the energy sector, with its traditional, classical solutions for smaller and larger amounts of energy. Energy storage has become of higher importance in relation with the development of alternative energy sources, called also renewable sources. The future energy systems will be based more and more on accumulation in the context of the development of so-called smart grids, or also in the Hybrid power plants. Energy storage devices are characterized by their density parameters, energy and power densities, but their efficiency must be analyzed carefully. A well-adapted tool has been proposed, namely "The Theory of Ragone Plots". Another simplified representation is called the MRR (Modified Ragone Representation).