NABCEP Solar PV System Design 4 — Questions and Answers
Question 1: What is the standard test condition (STC) irradiance level used when rating PV module output?
- 800 W/m²
- 1,000 W/m² (Correct answer)
- 1,200 W/m²
- 500 W/m²
Correct answer: 1,000 W/m²
STC specifies 1,000 W/m² irradiance, 25°C cell temperature, and AM 1.5 spectrum as the reference conditions for rating PV module output.
Question 2: A battery-based off-grid system needs to power a 1,200W load for 6 hours daily. What is the minimum daily energy storage requirement?
- 200 Wh
- 7,200 Wh (Correct answer)
- 1,200 Wh
- 3,600 Wh
Correct answer: 7,200 Wh
Daily energy requirement = 1,200W × 6 hours = 7,200 Wh, which sets the minimum usable battery capacity needed per day.
Question 3: When determining the number of modules per string for a string inverter, which inverter specification sets the upper boundary?
- Maximum DC input current
- Maximum DC input voltage (Correct answer)
- Minimum MPPT voltage
- Maximum AC output power
Correct answer: Maximum DC input voltage
The inverter's maximum DC input voltage (e.g., 600V or 1000V) caps the number of modules in series, as string Voc must not exceed this limit.
Question 4: What is the significance of a module's 'fill factor' (FF)?
- It measures how well the module converts heat to electricity
- It indicates how closely the module's I-V curve approaches an ideal rectangle (Correct answer)
- It defines the percentage of the module surface covered by cells
- It specifies the percentage of energy stored in the module
Correct answer: It indicates how closely the module's I-V curve approaches an ideal rectangle
Fill factor is the ratio of maximum power to the product of Voc and Isc, showing how 'square' the I-V curve is and reflecting overall cell quality.
Question 5: A grid-tied PV system must include anti-islanding protection. What is the primary reason for this requirement?
- To prevent overproduction and grid export
- To protect utility workers from energized lines during an outage (Correct answer)
- To reduce reactive power consumption
- To increase inverter efficiency
Correct answer: To protect utility workers from energized lines during an outage
Anti-islanding prevents the inverter from continuing to energize the grid during a utility outage, protecting lineworkers who may be repairing downed lines.
Question 6: In PVWatts or similar simulation tools, what does the 'system losses' percentage account for?
- Only inverter conversion losses
- All real-world derating factors including wiring, soiling, shading, and mismatch losses (Correct answer)
- The difference between STC and NOCT ratings
- Only temperature-related power reduction
Correct answer: All real-world derating factors including wiring, soiling, shading, and mismatch losses
System losses in tools like PVWatts represent a combined derate factor encompassing inverter, wiring, soiling, mismatch, shading, and other real-world losses.
Question 7: What is the purpose of including a DC disconnect switch between the PV array and the inverter?
- To regulate voltage output from the array
- To allow safe isolation of the DC source for maintenance and emergency shutoff (Correct answer)
- To convert DC power to AC power
- To provide overcurrent protection for the array
Correct answer: To allow safe isolation of the DC source for maintenance and emergency shutoff
The DC disconnect allows maintenance personnel and emergency responders to safely de-energize the DC side of the system without disturbing the inverter or AC side.
NABCEP Solar PV System Design 4
What is the standard test condition (STC) irradiance level used when rating PV module output?