NABCEP Solar Energy System Design & Installation 4 — Questions and Answers
Question 1: What is the main advantage of a bifacial PV module compared to a monofacial module?
- Lower temperature coefficient
- Ability to generate power from both front and rear surfaces (Correct answer)
- Higher rated Voc for the same cell count
- Built-in bypass diodes on both sides of the cell
Correct answer: Ability to generate power from both front and rear surfaces
Bifacial modules capture direct irradiance on the front and reflected/diffuse irradiance on the rear, increasing total energy yield.
Question 2: Under NEC 690.12, which rapid shutdown requirement applies to residential rooftop PV systems installed after the 2017 NEC adoption?
- The system must shut down within 30 seconds when activated
- Conductors in the array boundary must be de-energized to 30V or less within 30 seconds of initiating shutdown (Correct answer)
- The inverter must isolate the AC output only; DC conductors remain energized
- Rapid shutdown is only required for commercial installations over 10 kW
Correct answer: Conductors in the array boundary must be de-energized to 30V or less within 30 seconds of initiating shutdown
NEC 690.12 (2017+) requires that within the array boundary, conductors be reduced to 30V or less within 30 seconds of rapid shutdown initiation to protect first responders.
Question 3: What is the primary reason tilt angle optimization matters more for fixed-tilt systems in high-latitude locations than in equatorial regions?
- Solar irradiance is more intense at high latitudes
- The seasonal sun path variation is greater at high latitudes, so a steeper tilt better captures low winter sun (Correct answer)
- Equatorial regions have more cloud cover, making tilt irrelevant
- High-latitude systems use east-west orientation exclusively
Correct answer: The seasonal sun path variation is greater at high latitudes, so a steeper tilt better captures low winter sun
At high latitudes, the sun's elevation angle varies dramatically between summer and winter, so optimal tilt becomes more critical to capturing adequate winter energy.
Question 4: Which NABCEP competency category specifically addresses the ability to evaluate roof structure for PV mounting load capacity?
- Electrical Design
- Mechanical and Structural Design (Correct answer)
- Site Assessment
- System Commissioning
Correct answer: Mechanical and Structural Design
The Mechanical and Structural Design competency covers evaluating roof and mounting structure capacity, including dead loads, live loads, and wind uplift.
Question 5: A PV system's performance ratio (PR) is calculated as what?
- AC energy yield divided by the product of installed DC capacity and reference irradiation (Correct answer)
- DC energy divided by AC energy at the meter
- Inverter efficiency multiplied by wiring efficiency
- Annual irradiance divided by STC irradiance (1000 W/m²)
Correct answer: AC energy yield divided by the product of installed DC capacity and reference irradiation
PR = (AC energy output in kWh) / (Installed DC capacity in kWp × Reference irradiation in kWh/m²), representing overall system quality.
Question 6: What is the purpose of the 'derate factor' used in PVWatts and similar simulation tools?
- It adjusts for the difference between STC irradiance and actual irradiance
- It accounts for system losses such as wiring, soiling, inverter efficiency, and temperature to estimate real-world AC output (Correct answer)
- It corrects the panel's rated power for roof pitch
- It scales the system output for the local utility interconnection fee
Correct answer: It accounts for system losses such as wiring, soiling, inverter efficiency, and temperature to estimate real-world AC output
The derate factor multiplies the array's STC-rated power by a value less than 1.0 to account for all real-world efficiency losses between the panel and the utility meter.
Question 7: When designing a ground-mounted PV system, what is the primary reason for specifying module row spacing beyond minimum clearance?
- To comply with NEC minimum working space requirements around inverters
- To limit inter-row shading during low sun angle hours, especially in winter (Correct answer)
- To allow room for bifacial modules to access rear irradiance
- To meet fire department access path requirements
Correct answer: To limit inter-row shading during low sun angle hours, especially in winter
Adequate row spacing prevents the shadow of one row from falling on the next during critical morning/afternoon hours, maximizing annual energy production.
NABCEP Solar Energy System Design & Installation 4
What is the main advantage of a bifacial PV module compared to a monofacial module?