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, leading to the development of new technologies. The energy efficiency of the storage means is an important parameter, being often not considered in the conception and design of the applications. For the evaluation of the energetic performance of a storage device, a well-adapted tool has been proposed, namely "The Theory of Ragone Plots". This tool sets in evidence in what way the effectively recoverable energy amount of a device is depending on the power level of the charge/discharge process. Further, the taking into account of the power needed for the auxiliary equipment of a storage system like the circulation pumps of a flow battery, the vacuum pumps of a flywheel or the forced cooling of a battery can lead to a globally negative value of the efficiency.