Panels photovoltaic solar energy

What Is Balance System (BOS)?

The balance of system includes all the components of a photovoltaic system with the exception of photovoltaic panels.

What is balance system (BOS)?

We can think of a complete photovoltaic energy system made up of three subsystems.

  • On the power generation side, a subsystem of photovoltaic devices (cells, modules, arrays) converts sunlight into direct current (DC) electricity.
  • On the energy use side, the subsystem consists mainly of charging, which is the application of photovoltaic electricity.
  • Between these two, we need a third subsystem that allows the electricity generated by the photovoltaic modules to be applied correctly to the load. This third subsystem is often  called the "balance of system" or BOS .

A grid-connected system requires system balancing equipment that allows you to safely transmit electricity to your loads and meet the grid connection requirements of your energy provider. 

What Elements Make Up the Balance of System?

The balance of the system is, in general terms:

  • The electrochemical accumulator (or batteries). In the case of some systems isolated from the grid, the electricity generated cannot be supplied to the grid, in these cases it must be stored in batteries or similar elements.
  • The control unit.
  • The current inverter, which is electronic equipment that allows modulating and transforming the electrical current generated by the photoelectric plates.
  • Charge regulator.
  • The mechanical support structure.
  • The electrical wiring.
  • Protection devices (fuses, earthing, and switches).

Other optional components in a solar balance of system include:

  • Maximum power point tracking, MPPT
  • GPS solar tracker. In order to calculate the best inclination and orientation of the solar panels.
  • Energy management software. 
  • Solar concentrators. Concentrators allow solar radiation to be concentrated at a single point using mirrors.
  • Solar radiation sensors.
  • Wind equipment.
  • Specific task accessories designed to meet the specialized needs of a system owner.
  • In addition, CPV systems require lenses or optical mirrors, and sometimes a cooling system.

In addition, a large above-ground photovoltaic solar power station requires equipment and facilities, such as:

  • Network connections
  • Office and concrete installations.

The land where the plant is installed is sometimes included as part of the BOS as well.

How Much Does the Balance of System Cost?

The cost of the balance of system will include the cost of hardware (and software, if applicable), labor, which allows for interconnection and inspection fees, and any other fees that apply. For large commercial solar systems, the cost of BOS can include the cost of land and construction, etc.

To get an idea of ​​the cost of the balance of the photovoltaic system (solar BOS), the photovoltaic modules represent approximately 25% of the total cost. The cost of batteries is usually higher than that of photovoltaic panels, especially if periodic replacement is included.

So even if the modules were free, we would only reduce the cost of isolated stations by 25%.

Fortunately, many applications do not require accumulators (water pumping, irrigation, grid connection) and are free from this economic burden.

Charge Regulator

The charge regulator is a device that regulates the flow rates of electricity from the generation source to the battery and the charge.

The controller keeps the battery fully charged without overcharging it. When the load consumes power, the controller allows the load to flow from the generation source to the battery, the load, or both. When the controller detects that the battery is fully charged (or almost fully charged), it reduces or stops the flow of electricity from the generation source, or diverts it to an auxiliary or "bypass" charge (most commonly an electric water heater). ).

Many controllers will also detect when the loads have drawn too much power from the batteries and will stop the flow until sufficient charge is restored in the batteries. This last feature can greatly extend the life of the battery.

What Is the Balance of Plant?

Balance of plant (BOP) is a term generally used in the context of power engineering to refer to all supporting components and auxiliary systems of a power plant required to deliver power, in addition to the generating unit itself.

These can include transformers, inverters, support structures, etc., depending on the type of plant.


Published: May 13, 2015
Last review: February 13, 2019