AE 868
Commercial Solar Electric Systems

Lesson 5 Activity

Lesson 5 Activity
Activity Details

Scenario 1

Assume you are designing a system with the same module counts on the rooftop as shown in the video shown below. The Module used is Trina Solar Allmax 60 cell multicrystaline. You may choose the SOLARMOUNT rail based options that can serve the purpose 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 back fed to the lower side of the Main distribution panel (MDP), where all load circuits are connected, through a protection breaker . 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 and the PSF? (Hint: wind and snow load add weights. Dead load doesn't consider them. PSF is the per squre foot load)?
  • Based on the number of L-foot points you came up with, what is the Point Dead Load? (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.