Balance of System

We can think of a complete photovoltaic (PV) energy system as composed of three subsystems.

    * On the power-generation side, a subsystem of PV devices (cells, modules, arrays) converts sunlight to direct-current (DC) electricity.

    * On the power-use side, the subsystem consists mainly of the load, which is the application of the PV electricity.

    * Between these two, we need a third subsystem that enables the PV-generated electricity to be properly applied to the load. This third subsystem is often called the "balance of system," or BOS.

Illustration of the elements needed to get the power created by a PV system to the load (a house).

This simple illustration shows the elements needed to get the power created by a PV system to the load (in this example, a house). The stand-alone PV system (a) uses battery storage to provide dependable DC electricity day and night. Even for a home connected to the utility grid (b), PV can produce electricity (converted to AC by a power conditioner) during the day. The extra electricity can then be sold to the utility during the day, and the utility can in turn provide electricity at night or during poor weather.

The BOS typically consists of structures for mounting the PV arrays or modules and power-conditioning equipment that adjusts and converts the DC electricity to the proper form and magnitude required by an alternating-current (AC) load. The BOS can also include storage devices, such as batteries, so PV-generated electricity can be used during cloudy days or at night.