The Institute is Getting Solar Panels — Part III

We’ve been chronicling our addition of solar panels (see them all here). Today we’re going to go through the layout and electrical diagrams of the system.

The Layout

The panels will be mounted on the back of our garage. This is for a few reasons. First, our house’s roof slopes west–east while the garage is almost all a south face. Second, our home is part of a historic district and we prefer to add the panels to our nonhistoric garage, which is out of public view and won’t detract from the panorama of historic Cooksville.

In order to mount these panels on the garage roof, we had to replace a load-bearing beam built of plywood and 2-×-4-in lumber in the 1970s. It was not up to code and unclear if it would take the additional load of panels.

StructuralDrawing-Johnson-1.jpg

In this schematic drawing, the placement of three rows of solar panels can be seen on the roof. They are each tilted to 43°, equal to our latitude, which is the optimal angle for solar panels. Not only will this increase our overall solar radiation, it helps shed snow in the winter. They are spaced so that they do not shade one another. The conduit carrying two pairs of DC lines and a ground cable can be seen running from the roof, through the garage and into the ground.

StructuralDrawing-Johnson-2

From the overhead view, you can see how we are biasing our top row (left in the above image) away from the east side of the garage, as a tree shades this space. The rack supports are also visible, never spanning more than 4 ft. Each one is attached to the underlying rafters with lag screws. The entire foot is flashed to keep rain water from entering the roof structure.

ElectricLayout.gif

Inside the house, we have to connect the DC lines from the panels to the inverter. From there the power flows as AC to the electric panels of the house. Any extra power is shunted to the battery (as DC). If the power goes out, our on-site transformer provides us with grid-quality power for our back-up panel. All of this is monitored through a system meter. We are required to have an AC disconnect, severing power from the inverter to the electric panel so that our utility can service our line without worry of the system being energized from the solar side. This all results in a spaghetti monster of an electrical system, all connected with conduit.

The Electrical System

Aside from the structure, the solar system needs some complex electronics as it converts DC solar power to AC home current, stores DC power in a battery, and has monitoring and safety equipment.

One-Line-Drawing-Johnson

Our utility requires a “one-line diagram,” and the above drawing is one I created to satisfy this requirement. This draws a direct line between the solar panels and the grid power. In this case, we have two groups of eight panels wired in series. As each group generates up to 331 V and 9.5 A, we are using 10 gauge copper wire (THWN) as well as a 6-gauge ground. These will run through 3/4-in conduit from the roof to the house. From the inverter, DC power will be shunted to the battery, while AC will be split between a back-up panel — to power our essential utilities in the event of a power outage — and the main panel through an AC disconnect switch. Much of the interior AC wiring is 8 gauge. After our home power needs are met and the battery is full, any extra power will be fed back to the grid through our electricity meter.

WholesaleSolarDiagram-Johnson

Our solar panel provider also created a more technical diagram, where our conduit, wire gauge, and other information was listed for us. I have really enjoyed the electrical part of this project, much more than I thought I would.


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