NABCEP Module Technology 5 — Questions and Answers
Question 1: What is the difference between a module's 'nameplate efficiency' and its 'aperture efficiency'?
- Nameplate efficiency uses STC power while aperture efficiency uses NOCT power
- Nameplate efficiency is based on total module area including frame, while aperture efficiency uses only the active cell area (Correct answer)
- Nameplate efficiency applies to new modules, while aperture efficiency accounts for first-year LID losses
- Nameplate efficiency measures DC output, while aperture efficiency measures AC output after the inverter
Correct answer: Nameplate efficiency is based on total module area including frame, while aperture efficiency uses only the active cell area
Nameplate (module) efficiency divides Pmax by total module area including frame, while aperture efficiency uses only the light-collecting cell area, making aperture efficiency always higher than nameplate efficiency.
Question 2: Which phenomenon causes a string of modules to produce less current than a single unshaded module when one module in the string is partially shaded?
- The shaded module's bypass diodes redirect all current around the shaded string
- Series circuit current is limited to the lowest current module in the string, so partial shading of one module reduces current for all (Correct answer)
- Shaded modules develop a reverse voltage that reduces the shaded module's voltage contribution only
- Partial shading increases string resistance, causing a voltage drop across all modules equally
Correct answer: Series circuit current is limited to the lowest current module in the string, so partial shading of one module reduces current for all
In a series string, current must be the same through all modules; the most shaded module limits string current to its reduced Isc, causing disproportionate power loss across the entire string.
Question 3: What is the primary advantage of half-cut cell module technology compared to standard full-cell modules?
- Half-cut cells use thinner silicon wafers, reducing material cost by 50%
- Splitting cells in half reduces resistive (I²R) losses and minimizes the power loss from partial shading of one cell row (Correct answer)
- Half-cut cells have twice the voltage, allowing fewer series connections in a string
- Half-cut cells eliminate the need for bypass diodes in each module
Correct answer: Splitting cells in half reduces resistive (I²R) losses and minimizes the power loss from partial shading of one cell row
Half-cut cells carry half the current of full cells, reducing I²R losses by 75% in each cell (loss ∝ I²), and the module is internally split into two independent half-modules that reduce partial shade impact.
Question 4: For a grid-tied PV system in the US, which standard governs the electrical safety requirements that PV modules must meet?
- IEC 61215 — Design Qualification and Type Approval
- UL 61730 (formerly UL 1703) — Safety Standard for Photovoltaic Modules (Correct answer)
- IEEE 1547 — Standard for Interconnection of Distributed Energy Resources
- ASTM E1036 — Standard Test Methods for Electrical Performance of PV Cells
Correct answer: UL 61730 (formerly UL 1703) — Safety Standard for Photovoltaic Modules
UL 61730 (harmonized with IEC 61730) is the US safety standard covering module construction, insulation, fire resistance class, and electrical safety required for NEC-compliant grid-tied installations.
Question 5: What is 'snail trail' degradation in PV modules, and what causes it?
- Water infiltration paths visible under UV light caused by failed edge sealant
- Brown discoloration patterns on module glass caused by silver paste oxidation through micro-cracks in cells (Correct answer)
- Delamination lines from thermal expansion mismatches between glass and EVA
- Corrosion streaks on the module frame from salt mist exposure
Correct answer: Brown discoloration patterns on module glass caused by silver paste oxidation through micro-cracks in cells
Snail trails are brownish discoloration lines on the module glass that follow micro-cracks in cells; moisture penetrates cracks and oxidizes the silver cell metallization, causing visible marks and potential power loss.
Question 6: How does irradiance level affect the short-circuit current (Isc) and open-circuit voltage (Voc) of a PV module differently?
- Isc changes proportionally with irradiance; Voc changes logarithmically and much less (Correct answer)
- Both Isc and Voc change proportionally with irradiance, maintaining constant fill factor
- Voc changes proportionally with irradiance; Isc is relatively constant above 400 W/m²
- Isc increases while Voc decreases at higher irradiance due to cell heating
Correct answer: Isc changes proportionally with irradiance; Voc changes logarithmically and much less
Isc is directly proportional to irradiance (halve the sun, halve the current), while Voc changes only logarithmically — dropping roughly 5–10% at half irradiance versus 50% for Isc.
Question 7: A module's backsheet is rated as 'Type A' per IEC 61730. What does this fire performance classification require?
- The backsheet must be self-extinguishing within 60 seconds when exposed to flame
- The module must pass Class A fire spread tests equivalent to roofing material requirements, representing the highest fire resistance level (Correct answer)
- The backsheet must maintain electrical insulation at temperatures up to 150°C
- The module must withstand a 12.7 mm hailstone impact at 23 m/s without backsheet cracking
Correct answer: The module must pass Class A fire spread tests equivalent to roofing material requirements, representing the highest fire resistance level
IEC 61730 Application Class A (fire rating) requires the module to meet the most stringent fire resistance criteria, equivalent to UL Class A roofing — required for residential rooftop installations in most US jurisdictions.
NABCEP Module Technology 5
What is the difference between a module's 'nameplate efficiency' and its 'aperture efficiency'?