Accurate modeling of these storage systems is essential for their optimal integration into smart grids and hybrid renewable energy setups. At the heart of ESS modeling lies the mathematical formulation of the energy balance equation, which serves as the central structure that enables dynamic and realistic simulations of various storage technologies. This equation quantifies the energy flow, accounting for input, output, storage, and losses, allowing precise prediction of system behavior under changing operating conditions [1–5]. In this chapter, we start with the fundamentals (energy balance and SOC), and then focus on the physics of each technology: power cells (from empirical to CFD-level detail), batteries (electrical/electrochemical/thermal and hybrid models), supercapacitors (from ideal C to transmission-line models), and flywheels (mechanical, electromechanical, and multiphysics). In this chapter, readers will learn how models couple to power electronic interfaces and how to apply them for simulation studies and control design using MATLAB/Simulink. By aligning the modeling approach with the energy balance formulation, the chapter provides the essential tools for understanding, designing, and optimizing ESS to support renewable energy adoption effectively (Fig. 5.1).