Part 4 – Balance-of-System

This is Part 4 in the “PV Solar – Homeowner Analysis” series, comparing the cost of fossil fuel generated electricity used in my home with the cost of electricity generated by a theoretical photovoltaic system. In part one we went through the process of sizing a system, in part two we continued by examining how this type of system would work with the existing utility grid and in part three we covered solar panels (modules), which make up approximately 50-60% of the installed cost of the system. In this fourth segment I’ll cover the remaining elements making up a grid tied residential solar electric system.

The remaining elements of a solar electric system are called Balance-of-System (BOS) components. The major BOS elements include such items as the grid Tie inverter, the combiner box, the AC & DC disconnects, the module mounting system and the cable and wiring. There are other optional parts, but these elements make up the major BOS components of a residential grid tie system.

• Grid Tie Inverter
  The solar inverter converts the variable DC output of the photovoltaic cells into AC current. The grid tie inverter is a pure sine wave inverter that synchronizes with and feeds power back to the grid. A grid tie inverter connects directly to the utility grid without the use of batteries. One important thing to consider when purchasing the inverter is the efficiency. Even though you may have purchased solar panels with a very high PTC wattage rating, a low efficiency rated inverter can negatively effect the system’s performance.




• Combiner Box
  The Combiner box is where all the wires from the Solar Panels are routed and allows multiple solar panels to be combined in parallel. A combiner box is basically an electrical box that connects the input panels in parallel to produce one circuit. The breakers and fuses for the solar panels will also be placed here as well. The box is typically made to go outside near the solar panels.




• AC & DC Disconnects
  The DC disconnect is used to isolate the PV array from the inverter and an AC disconnect is used to isolate the inverter from the AC panel or load center. The AC disconnect (switch), installed on the inverter output, allows the system to be disconnected from the grid by utility personnel.




• Module Mounting System
  After you’ve establish the number of panels and the array arrangement, the mounting location is determined. Although module mounting systems are available for ground and roof installation, roof mount installation is the most common and cost effective method. The roof mounted panels are attached to a mounting system typically consisting of an aluminum or steel support structure which attaches the panels to the roof. If the roof space is limited, shaded or unsatisfactory for any reason, the array can be mounted on the ground. The most common type of ground mount is a wedge shaped steel structure anchored in concrete. Another type of ground mounted system is the pole mount, in which the panel array is mounted on top of a single steel pole. There are numerous variations of these installation types, including rail mounts, tilt adjustment, automatic tracking, etc.




• Cables and Wiring
  This includes the Wiring that connects the components of the PV system together. Most solar panels are now manufactured with multi-contact connectors (MC) which makes series and parallel connections a snap. The panels are wired together as needed and then routed into the combiner box paying special attention to safety and grounding requirements.

That’s it for the Balance-of-System components. In the next post I’ll go over the component and installation costs along with the Federal and State tax incentives and rebates to come up with a fixed cost for the estimated life of the system. As I stated before, this is a learning experience, so if you see any errors or omissions, please feel free to share and leave a comment.

Before I end this segment, I want to refer back to part 1 and revise the calculated system size. The last step used in determining the size was to multiply by a real-world factor of 1.2. Further study suggests that a factor of 1.3 is actually closer to a real-world situation. Due to module and inverter inefficiencies along with other power losses, the calculated system size of the 4.271 kW system is multiplied by a factor of 1.3 to give a more real-world estimate (4.27 kW x 1.3 = 5.55 kW) of 5.55 kW.


Recommended Site
Pennsylvania Solar Course
This is an online solar training course from the state of Pennsylvania designed for residential applications and specifically includes the siting, sizing and installation of solar electric (PV) and solar water heating.