NABCEP Solar PV System Design 5 — Questions and Answers
Question 1: A PV system designer is selecting between monocrystalline and polycrystalline modules for a space-constrained rooftop. Which module type is generally preferred and why?
- Polycrystalline, because it costs less per watt
- Monocrystalline, because it offers higher efficiency and greater power density per area (Correct answer)
- Thin-film, because it handles shading better
- Polycrystalline, because it has a better temperature coefficient
Correct answer: Monocrystalline, because it offers higher efficiency and greater power density per area
Monocrystalline modules achieve higher efficiency (typically 19-23%) allowing more power to fit in a limited roof area despite their higher cost per module.
Question 2: What is the purpose of a rapid shutdown system (RSS) in a rooftop PV installation per NEC 690.12?
- To rapidly increase output power during peak demand
- To reduce PV system conductors to a safe voltage level within the array boundary for firefighter safety (Correct answer)
- To automatically restart the system after a grid outage
- To disconnect the utility meter during high irradiance
Correct answer: To reduce PV system conductors to a safe voltage level within the array boundary for firefighter safety
NEC 690.12 requires rapid shutdown to de-energize conductors within the array boundary to a safe voltage within 30 seconds, protecting firefighters from electric shock.
Question 3: When using microinverters instead of a string inverter, what key advantage is gained for systems with partial shading?
- Higher peak DC voltage
- Each module operates at its own maximum power point, minimizing shade impact on unshaded modules (Correct answer)
- Simpler NEC compliance for rapid shutdown
- Lower upfront equipment cost
Correct answer: Each module operates at its own maximum power point, minimizing shade impact on unshaded modules
Microinverters perform MPPT at the individual module level, so shading on one module does not drag down the output of unshaded modules in the array.
Question 4: A designer is sizing conductors for a PV source circuit. The module Isc is 9.5A. Per NEC 690.8, what minimum ampacity must the conductors be rated for?
- 9.5A
- 11.9A (Correct answer)
- 14.3A
- 7.6A
Correct answer: 11.9A
NEC 690.8(A) requires PV source circuit conductors to be rated for 125% of module Isc: 9.5A × 1.25 = 11.875A, rounded up to 11.9A minimum.
Question 5: In a battery energy storage system (BESS) paired with PV, what does 'depth of discharge' (DoD) refer to?
- How deep underground the battery bank is installed
- The percentage of total battery capacity that is used before recharging (Correct answer)
- The voltage difference between full charge and empty states
- The rate at which the battery discharges in ampere-hours
Correct answer: The percentage of total battery capacity that is used before recharging
Depth of discharge describes what fraction of total battery capacity is actually used; operating at lower DoD generally extends battery cycle life.
Question 6: Which production estimation method uses satellite-derived irradiance data and is commonly accepted in NABCEP best practices?
- Manual sun-hours calculation from local newspaper tables
- TMY (Typical Meteorological Year) datasets used in software tools like PVWatts or SAM (Correct answer)
- Direct measurement with a handheld lux meter for one week
- Manufacturer-provided regional yield estimates only
Correct answer: TMY (Typical Meteorological Year) datasets used in software tools like PVWatts or SAM
TMY datasets compiled from satellite and ground station data are the NABCEP-recognized standard for production estimation in tools like PVWatts and SAM.
Question 7: A grid-tied 10 kW AC system has an annual production of 13,500 kWh. What is its capacity factor?
- 7.5%
- 15.4% (Correct answer)
- 25.0%
- 13.5%
Correct answer: 15.4%
Capacity factor = annual production / (rated power × 8,760 hours) = 13,500 / (10,000 × 8,760) × 100 ≈ 15.4%.
NABCEP Solar PV System Design 5
A PV system designer is selecting between monocrystalline and polycrystalline modules for a space-constrained rooftop.
Which module type is generally preferred and why?