NABCEP Solar Site and Shading Analysis 3 — Questions and Answers
Question 1: A designer uses the 'worst-month' method for a standalone PV system in Colorado. What is the purpose of identifying the worst month?
- To size conductors for peak summer current
- To size the battery bank and array for the month with the least solar resource relative to load (Correct answer)
- To determine the maximum open-circuit voltage in coldest conditions
- To schedule module cleaning for the lowest irradiance period
Correct answer: To size the battery bank and array for the month with the least solar resource relative to load
The worst month is when the ratio of available solar energy to the system load is smallest, ensuring the system is sized to meet demand even under the most challenging solar conditions.
Question 2: True south in the continental US differs from magnetic south because of magnetic declination. In Seattle, WA, where declination is approximately 15°E, a compass-south-facing array is actually oriented:
- 15° west of true south
- 15° east of true south (Correct answer)
- Exactly at true south
- 15° above the horizon
Correct answer: 15° east of true south
An easterly declination of 15° means magnetic north is 15° east of true north, so a compass-south reading points 15° east of true south.
Question 3: During a shading analysis consultation, a client insists on keeping a large oak tree that shades the array from 8–9 AM year-round. The designer should explain that morning shading primarily reduces:
- Peak power output at solar noon
- Early morning energy production but has minimal impact on total daily kWh in most US locations (Correct answer)
- The system's nighttime battery reserve
- Module temperature coefficients throughout the day
Correct answer: Early morning energy production but has minimal impact on total daily kWh in most US locations
Early morning irradiance is relatively low compared to midday, so shading during the first hour after sunrise reduces total daily energy production by a proportionally small amount in most temperate US locations.
Question 4: What is the 'sky diffuse' component of solar radiation, and why is it important in shading analysis?
- Direct beam radiation from the solar disk only
- Radiation scattered by the atmosphere arriving from all parts of the sky dome, which is reduced by nearby obstructions blocking portions of the sky (Correct answer)
- Radiation reflected upward from the ground surface
- Radiation that bypasses the atmosphere via refraction
Correct answer: Radiation scattered by the atmosphere arriving from all parts of the sky dome, which is reduced by nearby obstructions blocking portions of the sky
Sky diffuse radiation comes from the entire sky dome; obstructions that block portions of the sky reduce diffuse irradiance in addition to blocking direct beam radiation, making full 3D shading analysis important.
Question 5: A roof-mounted array has a 2% annual shading loss according to a simulation. If the system produces 18,000 kWh/year without shading, what is the estimated annual production WITH shading?
- 17,640 kWh (Correct answer)
- 18,360 kWh
- 16,200 kWh
- 17,820 kWh
Correct answer: 17,640 kWh
A 2% shading loss means production = 18,000 × (1 − 0.02) = 18,000 × 0.98 = 17,640 kWh per year.
Question 6: Which of the following describes the concept of 'far shading' in solar PV analysis software like PVsyst?
- Shading caused by module rows within the same array on each other
- Shading from distant objects (mountains, hills, large buildings) that affect the horizon profile (Correct answer)
- Shading losses caused by soiling and bird droppings
- Self-shading of a module by its own frame at low sun angles
Correct answer: Shading from distant objects (mountains, hills, large buildings) that affect the horizon profile
Far shading refers to obstruction from distant objects that alter the effective horizon line, distinct from near shading caused by close objects like adjacent rows or roof structures.
Question 7: A solar installer is evaluating two roof sections: one with 20% annual shading loss and one with 4% annual shading loss. Both sections have identical usable area. Which section should be prioritized and why?
- The 20%-loss section, because more shade keeps modules cooler and improves efficiency
- The 4%-loss section, because lower shading produces significantly more annual energy per unit area (Correct answer)
- Both sections perform equally since shade only matters at peak hours
- The 20%-loss section, because DC optimizers eliminate all shading losses
Correct answer: The 4%-loss section, because lower shading produces significantly more annual energy per unit area
The section with only 4% annual shading loss will produce substantially more energy because a higher fraction of available solar resource reaches the modules throughout the year.
NABCEP Solar Site and Shading Analysis 3
A designer uses the 'worst-month' method for a standalone PV system in Colorado.
What is the purpose of identifying the worst month?