Simply
put, PV systems are like any other electrical power generating
systems, just the equipment used is different than that used for
conventional electromechanical generating systems. However, the
principles of operation and interfacing with other electrical
systems remain the same, and are guided by a well-established body
of electrical codes and standards.
Although a PV array produces power when exposed to sunlight, a
number of other components are required to properly conduct,
control, convert, distribute, and store the energy produced by the
array.
Depending on the functional and operational requirements of the
system, the specific 
components required, and may include major
components such as a DC-AC power inverter, battery bank, system
and battery controller, auxiliary energy sources and sometimes the
specified electrical load (appliances). In addition, an assortment
of balance of system (BOS) hardware, including wiring, over current,
surge protection and disconnect devices, and other power
processing equipment. The Figure above show a basic diagram of a
photovoltaic system and the relationship of individual components.
Why
Are Batteries Used in Some PV Systems?
Batteries are often used in PV systems for the purpose of storing
energy produced by the PV array during the day, and to supply it
to electrical loads as needed (during the night and periods of
cloudy weather). Other reasons batteries are used in PV systems
are to operate the PV array near its maximum power point, to power
electrical loads at stable voltages, and to supply surge currents
to electrical loads and inverters. In most cases, a battery charge
controller is used in these systems to protect the battery from
overcharge and over discharge
