NABCEP PV System Design Principles 2 — Questions and Answers
Question 1: When sizing a PV array for a battery-based off-grid system, which factor most directly determines the minimum array size?
- Peak sun hours at the site (Correct answer)
- The inverter's surge rating
- Battery float voltage
- Module temperature coefficient
Correct answer: Peak sun hours at the site
Peak sun hours represent the equivalent hours of full irradiance and directly determine how much energy the array can produce per day.
Question 2: A PV system is designed with modules having a temperature coefficient of Pmax of -0.35%/°C. If STC is 25°C and the cell temperature reaches 65°C, by what percentage does output power decrease?
- 14% (Correct answer)
- 22.75%
- 8.75%
- 3.5%
Correct answer: 14%
The temperature rise is 65 - 25 = 40°C, and 40 × 0.35% = 14% power reduction.
Question 3: In a grid-tied PV system, the primary function of the inverter is to:
- Convert DC power from the array to AC power for the grid (Correct answer)
- Store excess energy in batteries
- Step up the array voltage to transmission levels
- Regulate battery state of charge
Correct answer: Convert DC power from the array to AC power for the grid
The inverter converts the DC output of the PV array into AC power compatible with the utility grid.
Question 4: Which NEC article primarily governs the installation of photovoltaic systems?
- Article 690 (Correct answer)
- Article 705
- Article 702
- Article 480
Correct answer: Article 690
NEC Article 690 specifically covers solar photovoltaic systems and their installation requirements.
Question 5: The 'fill factor' (FF) of a PV module is best described as:
- The ratio of maximum power to the product of Voc and Isc (Correct answer)
- The ratio of active cell area to total module area
- The percentage of incident light converted to electricity
- The number of cells in series divided by total cell count
Correct answer: The ratio of maximum power to the product of Voc and Isc
Fill factor = Pmax / (Voc × Isc) and measures how close the I-V curve approaches an ideal rectangle.
Question 6: When designing a string of PV modules for a string inverter, the maximum string voltage must not exceed:
- The inverter's maximum DC input voltage rating (Correct answer)
- The combined Vmp of all modules
- 690 volts per NEC requirements
- The utility grid voltage multiplied by 1.25
Correct answer: The inverter's maximum DC input voltage rating
The maximum string open-circuit voltage at minimum expected temperature must stay within the inverter's rated maximum DC input voltage.
Question 7: Which shading scenario causes the greatest power loss in a series-connected PV string?
- One module completely shaded (Correct answer)
- Uniform light shading across all modules
- Partial shading of the bottom row only
- Shading of only one cell in each module
Correct answer: One module completely shaded
In a series string, one fully shaded module becomes a current-limiting bottleneck that can reduce the entire string's output drastically.
NABCEP PV System Design Principles 2
When sizing a PV array for a battery-based off-grid system, which factor most directly determines the minimum array size?