AE 868
Commercial Solar Electric Systems

Lesson 5 Activity

Lesson 5 Activity
Activity Details

Scenario 1

Assume you are designing a system with same module counts on the roof top as shown in the video shown below. The Module used is Trina Solar Allmax 60 cell multicrystaline. You have a rail that is 120” long from Unirac (a racking manufacturer). You have standard 12” spaced rafters for a residential roof.

Video: How Solar Works SSI (0:58)

How Solar Works SSI
Click for text description of How Solar Works

As shown in the video, a solar array can be installed on a rooftop for residential application. After all mechanical holding structures are installed on the roof, the solar panels can be fastened to it . When the sun shines on PV panels, photons are converted into electrons that form electrical current when it moves towards the circuit terminals . The electrical DC current then travels through conduits down towards a power conditioning unit (AKA inverter) . The current is converted to AC electrical current and travels through an AC disconnect box . When the AC current exits the disconnect box, it goes through a PV production meter to count energy generated . After that, the current is backfed to the lower side of the Main distribution panel (MDP), where all load circuits are connected, through a protection breakers . When the PV array is generating power, the load circuits draw power from the PV array instead of the grid . While the PV array is generating power, the monitoring device is transmitting the data to an online portfolio, where clients can see their instantaneous PV array performance online . Finally, the excess energy generated from the PV array is returned to the utility grid .

Credit: Sam Fortune
  • Find how many rails, stand-offs with L-foot, splices, mid clamps, and end clamps are required.
  • What is the dead structural load (Hint: wind and snow load add weights. Dead load doesn't considers them)?
  • Based on the number of L-foot points you came up with, what is the point PSF? (Hint: the dead load will be divided on the roof rafter based on the number of attachment point).
  • In case the roof is built with trusses spaced at 24” (instead of 12"). How does this affect the attachments to the PV system in terms PSF? (Hint: use the analogy of a table with three legs instead of four!)

Scenario 2

Assuming the roof is flat, the installation to flat roof with ballasted system from Unirac, taking the average cement weight of 32 lb. and considering the following:
  • Wind speed for State College, PA to be 90 mph

  • Exposure category to be "B"

  • Roof height is 15' for residential installations
  • How many concrete blocks do you need per module to have a safe installation?

  • Hints for both Scenarios:
    • Design with safety factor according to the manufacturer's specification.
    • Use UNIRAC DESIGN TOOL (create a free account to access it)


Prepare a report that includes your findings for each of the scenarios. The report should be no more than one double-spaced page in a 12 point font.

Submission Instructions and Grading Please visit the Lesson Activity page for submission instructions and grading information.