In recent years, electrical energy storage (EES) became the most important technology in isolated power systems (IPS), electrical power quality, distributed energy resources, and sustainable mobility. Electric Energy Storage Systems explains the modeling, planning, and implementation methodologies of EES systems. The book presents an overview of the technical and regulatory boundaries on the evolution of electric power systems to future smart grids, including concepts such as power to gas and power to heat. Distribution networks need storage due to volatile, low duty -cycle renewable energy sources (RES), such as solar and wind, to increase green energy penetration, while maintaining power quality and energy resilience.