NABCEP Solar Site and Shading Analysis 5 — Questions and Answers
Question 1: A site assessment reveals that a proposed rooftop array will have a solar access value of 82% annually. According to NABCEP guidelines, how should this value be used in client communications?
- Report it as 100% since bypass diodes eliminate all shading losses
- Use it to calculate a derated energy production estimate, clearly communicating that shading will reduce output by 18% compared to an unshaded system (Correct answer)
- Advise the client that 82% access means the system will only work 82% of daylight hours
- Apply it only to the DC system sizing, not the AC energy production estimate
Correct answer: Use it to calculate a derated energy production estimate, clearly communicating that shading will reduce output by 18% compared to an unshaded system
Solar access of 82% means 18% of potential solar energy is blocked by shading; this factor should be applied to the unshaded energy estimate to give the client an accurate production projection.
Question 2: Which of the following site conditions would most benefit from using microinverters or DC power optimizers compared to a standard string inverter configuration?
- A south-facing unobstructed roof with no shading year-round
- A complex roofline with modules on multiple orientations and partial shading from a chimney (Correct answer)
- A ground-mount array with all modules at identical tilt and azimuth
- A large commercial flat roof with uniform east-facing tilt and no obstructions
Correct answer: A complex roofline with modules on multiple orientations and partial shading from a chimney
Microinverters and DC optimizers provide module-level MPPT, which is most beneficial when modules experience unequal shading or different orientations that would otherwise cause severe mismatch losses in a string.
Question 3: When using the Solmetric SunEye for shading analysis, the device is placed at the array plane and photographs the sky. The resulting 'TOF' (Table of Obstructions) reports shading by:
- Watt-hours per square meter blocked per day
- Monthly and annual solar access percentages accounting for both beam and diffuse shading (Correct answer)
- Number of hours per day the array is completely shaded
- Peak irradiance blocked in W/m² during the summer solstice
Correct answer: Monthly and annual solar access percentages accounting for both beam and diffuse shading
The SunEye TOF report provides monthly and annual solar access percentages that factor in both direct beam shading from obstructions and the reduction in diffuse sky radiation from blocked sky dome portions.
Question 4: A NABCEP-certified designer finds that the proposed array location has only 68% annual solar access due to mature trees. According to best practices, what should the designer do FIRST before recommending system downsizing?
- Immediately file a solar easement with the county recorder
- Evaluate whether tree trimming, removal, or alternative mounting locations could improve solar access above an acceptable threshold (Correct answer)
- Install the system and apply for a shading variance from the utility
- Recommend a battery storage system to compensate for the shading losses
Correct answer: Evaluate whether tree trimming, removal, or alternative mounting locations could improve solar access above an acceptable threshold
Before accepting poor solar access, the designer should explore mitigation options such as selective tree trimming, removal, or relocating the array to an area with better solar access.
Question 5: In solar resource data analysis, the term 'P90' production estimate means:
- The system will produce at 90% of its rated nameplate capacity
- There is a 90% probability that actual annual production will meet or exceed this energy estimate (Correct answer)
- The system operates at 90% efficiency under standard test conditions
- 90% of modules in the array will be unshaded at any given time
Correct answer: There is a 90% probability that actual annual production will meet or exceed this energy estimate
P90 is a financial risk metric meaning there is a 90% probability (high confidence) that the system will produce at least this much energy annually, accounting for weather variability and uncertainty in the resource data.
Question 6: A shading analysis shows that an obstruction causes 'hard shading' on one module during peak hours from November through January. To minimize the annual energy impact, which module layout strategy is MOST effective?
- Place the shade-affected module in the center of the longest string
- Isolate the shade-affected module on its own dedicated string or optimizer circuit (Correct answer)
- Use a module with a lower temperature coefficient in the shaded position
- Increase the number of bypass diodes in unshaded modules
Correct answer: Isolate the shade-affected module on its own dedicated string or optimizer circuit
Isolating the shade-affected module on its own string or DC optimizer allows it to operate at its own MPPT, preventing its reduced output from dragging down all other modules sharing the same string.
Question 7: Which standardized dataset is MOST commonly used in the US for hourly solar resource data in PV system performance simulations?
- ECMWF ERA5 reanalysis global model output
- NREL's National Solar Radiation Database (NSRDB) TMY or PSM data (Correct answer)
- NASA POWER global irradiance averages
- NOAA hourly surface observation station records
Correct answer: NREL's National Solar Radiation Database (NSRDB) TMY or PSM data
NREL's NSRDB provides Typical Meteorological Year (TMY) and Physical Solar Model (PSM) data widely used in US PV simulation tools such as PVWatts, SAM, and PVsyst for bankable energy production estimates.
NABCEP Solar Site and Shading Analysis 5
A site assessment reveals that a proposed rooftop array will have a solar access value of 82% annually.
According to NABCEP guidelines, how should this value be used in client communications?