Balance of System


Wiring is used to transfer electricity throughout the system. Each section of wire needs to be sized in order to withstand the maximum voltage and is sized in gauges. The gauge number sets the size of the wire and can be chosen through the applicable NEC codes.

Source: wire craft



A meter is used to measure the amount of electricity produced by the solar panels and also received from the grid.  A common type of meter used is a small induction motor that drives gears and is calibrated to count kWh. Meters either have to be manually inspected to be bill, but modern meters have the ability to transmit this back to utilities.

With the advent of solar energy new meters were designed that are able to measure electricity both ways in order for the home owner to get credit for the energy they delivered back to the grid.


Junction Box

A junction box collects numerous different wires and transfers them into a common conduit sheath. A junction box is used to protect the wiring and ensure integrity of the circuit. They are fairly low cost but an integral part in a solar system.

The junction box has to be rated for the maximum voltage according to NEC codes.

Contributed to Pete LaLoggia


AC and DC Disconnect

Disconnects are essentially an electrical switch. They can be used to isolate the various parts of the system from one another. They are required on any solar system that is tied into the electric grid and are necessary to work on any piece of equipment in the system safely. They are designed to meet NEC code and have to be able to withstand the maximum voltage in the system.


Breaker box

The breaker box contains the fuses that protects the house. All electricity that flows into the house has to go through the breaker box to be up to electrical code.

The main electricity flows into the box, where it is split up to the separate circuits within the house. This is the reason why when a fuse trips only part of the electrical circuit of the house shuts down.

Solar installations have to be placed on a separate circuit due to the potential for a high voltage output. Without this housing and fuses, the home electrical circuit could be destroyed, potential leading to fires.

In order to be attached the electrical grid, the PV wiring through the breaker box has to be up to the incumbent utility’s interconnection standards. This can be posted on the utility’s website or can be found by contacting the utility.


Mounting Systems

Mounting systems are used to attach the solar modules to a roof or a racking system. Mounting systems are generally made of aluminum that been galvanized to prevent corrosion from the weather. The mounts are attached to the roof generally with screws and are shaped like a u-channel for ease of bolting the solar modules in.

Modules are mounted roughly six inches off the roof to allow air to flow underneath the module to allow for cooling. Mounting systems can come in various shapes and sizes depending on the manufacturer. Before mounting the modules to the roof, a roof inspection needs to be undertaken to make sure the roof can structurally stand the modules. Wind loads have to be taken into account due to airfoil affects when wind flows underneath the modules. Mounting systems can only be put in certain sections of the roof because an area roughly 3 feet from the edge of the roof experiences wind speed increases.


‘Soft’ Infrastructure

Soft infrastructure refers to the parts of the solar system not associated with physical pieces of equipment. These include installation labor, financing costs, taxes, permitting fees, and staff overhead for installation companies. Because of the rapidly decreasing price of the equipment in solar systems, focus on cost reductions has shifted to these soft cost for improvements.

A streamlines permitting process allows for less time and labor to be used for making sure the system is up to code.  Sales tax incentives can be implemented at a local level to reduce the cost to the system.

Financing cost is variable to each individual customer. The price to access capital changes with interest rates, funding sources, credit, and financing methods.


Breakdown of Cost

The breakdown of costs can be shown in the following image in 2012 in the United States. The total $/Watt for a solar installation depends on many factors including size of system, market prices, labor prices, and permitting steps need to install.

The cost of PV installation and materials has continued to drop due to advances in supply chain and experience with installation of solar systems. Economies of scale allow for larger solar systems to be cheaper than smaller ones because of fixed costs.

Source: DOE,