For practical applications, a large number of solar cells are interconnected and
encapsulated into units called PV modules, which is the product usually sold to the
customer. They produce DC current that is typically transformed into the more useful AC
current by an electronic device called an inverter. The inverter, the rechargeable batteries
(when storage is needed), the mechanical structure to mount and aim (when aiming is
necessary) the modules, and any other elements necessary to build a PV system are called
the balance of the system (BOS). These BOS elements are presented in Chapters 17 to 19.