Even if Electrochemical Storage (ES) devices are nowadays commonly used in a wide range of applications of different power, one application cannot be clearly addressed by only one specific technology of ES. Moreover, operating conditions such as charge/discharge rates or ambient temperatures have a huge influence on both ES real-time performance and lifetime. In this article, a new indicator of the energetic reserve, the State-of-Energy (SoE), is proposed to deal with modern Battery Management Systems (BMS) attendees: easy-to-implement, reliable given the operating conditions, using the power as a direct input parameter to manage the battery reserve. After a complete description of the SoE and the experimental characterization procedures, the paper presents some detailed examples of the SoE applications like storage technology comparison, smoothing of solar or wind production and finally on-line forecasting in advanced applications such as smart grids or electrical vehicle. (C) 2012 The Electrochemical Society. [DOI: 10.1149/2.075208jes] All rights reserved.