NABCEP PV System Design Principles 4 — Questions and Answers
Question 1: In a battery-based PV system, the depth of discharge (DOD) limit for lead-acid batteries is typically set at 50% to:
- Extend battery cycle life and prevent sulfation damage (Correct answer)
- Increase the usable capacity of the battery bank
- Reduce the required array size
- Comply with NEC requirements for battery systems
Correct answer: Extend battery cycle life and prevent sulfation damage
Limiting DOD to 50% significantly extends lead-acid battery cycle life by reducing sulfation and plate degradation.
Question 2: Which performance ratio (PR) range is considered typical for a well-designed, operational grid-tied PV system?
- 0.75 to 0.85 (Correct answer)
- 0.50 to 0.65
- 0.90 to 0.99
- 0.40 to 0.55
Correct answer: 0.75 to 0.85
A performance ratio of 0.75–0.85 accounts for typical real-world losses including temperature, wiring, inverter efficiency, and soiling.
Question 3: The 'clipping' loss in a PV system occurs when:
- The array's DC output exceeds the inverter's maximum AC output capacity (Correct answer)
- Module voltage exceeds the inverter's MPPT range
- Tree shadows intermittently cover portions of the array
- The grid voltage rises above normal limits
Correct answer: The array's DC output exceeds the inverter's maximum AC output capacity
Clipping happens when irradiance is high enough that the array produces more DC power than the inverter can convert, causing the inverter to limit output.
Question 4: The DC-to-AC ratio (also called inverter loading ratio or ILR) of 1.2 for a PV system means:
- The DC nameplate capacity is 1.2 times the inverter's AC output rating (Correct answer)
- The system produces 1.2 times more energy than it consumes
- There are 1.2 modules per inverter input
- The DC voltage is 1.2 times the AC voltage
Correct answer: The DC nameplate capacity is 1.2 times the inverter's AC output rating
An ILR of 1.2 means the array's STC DC capacity is oversized by 20% relative to the inverter's rated AC output, which is common to optimize annual energy yield.
Question 5: An azimuth angle of 180° for a fixed-tilt PV array in the northern hemisphere indicates the array faces:
- Due south (Correct answer)
- Due north
- Due east
- Due west
Correct answer: Due south
In the northern hemisphere, true south (180° azimuth) maximizes annual solar irradiance for a fixed-tilt array.
Question 6: Which type of mounting system is most appropriate for flat commercial rooftops to minimize roof penetrations?
- Ballasted racking systems (Correct answer)
- Direct roof-attached rail systems
- Ground-mounted post systems
- Flush-mount clip systems
Correct answer: Ballasted racking systems
Ballasted racking uses weight (concrete blocks or the racking weight itself) to hold arrays in place without penetrating the roofing membrane.
Question 7: When using PVWatts or similar simulation tools, the 'system losses' input of approximately 14% typically accounts for:
- Wiring, soiling, shading, mismatch, inverter, and other real-world efficiency losses (Correct answer)
- Only the inverter conversion efficiency loss
- Module degradation over the system lifetime
- Installation labor and material overhead costs
Correct answer: Wiring, soiling, shading, mismatch, inverter, and other real-world efficiency losses
The default ~14% system losses in PVWatts aggregates multiple derate factors including soiling, wiring resistance, mismatch, shading, inverter efficiency, and availability.
NABCEP PV System Design Principles 4
In a battery-based PV system, the depth of discharge (DOD) limit for lead-acid batteries is typically set at 50% to: