Energy storage has been a part of many PV systems since the beginning, but now the market is growing like never before. In PV203 the focus is on the fundamentals of battery­-based PV systems. The applications and configurations are many, and their complexity far exceeds that of grid­-direct PV systems. Components such as batteries, charge controllers, and battery­-based inverters are covered in detail, along with safety and maintenance considerations unique to battery­-based systems. Load analysis is critical to system design and will also be addressed along with other design criteria such as battery bank configuration and the electrical integration of the system.