NABCEP Solar Site and Shading Analysis 4 — Questions and Answers
Question 1: When measuring roof azimuth with a compass for solar site assessment, which correction must be applied to obtain the true azimuth?
- Add the site latitude to the compass reading
- Apply magnetic declination correction for the geographic location (Correct answer)
- Subtract the roof tilt angle from the compass bearing
- Add 180° to convert from bearing to solar azimuth convention
Correct answer: Apply magnetic declination correction for the geographic location
Compass measurements read magnetic azimuth; applying the local magnetic declination (adding for westerly, subtracting for easterly) converts the reading to true geographic azimuth needed for accurate solar modeling.
Question 2: An array installed at 35°N latitude uses a fixed tilt equal to the site latitude (35°). To maximize annual energy production, what would be the optimal adjustment to this tilt for a summer-peak load application?
- Increase tilt to 55° to capture more morning sun
- Decrease tilt to approximately 20–25° to better capture high summer sun angles (Correct answer)
- Keep tilt at 35° since latitude-tilt is always optimal for summer
- Increase tilt to 90° for vertical mounting
Correct answer: Decrease tilt to approximately 20–25° to better capture high summer sun angles
A tilt lower than site latitude (roughly latitude minus 10–15°) optimizes for summer when the sun is high; latitude tilt optimizes for annual average, not specifically summer production.
Question 3: A homeowner's neighbor plants a row of fast-growing Leyland cypress trees. What site assessment tool or documentation should a solar professional recommend creating NOW, before the trees grow?
- A ground-mount permit application
- A baseline shading analysis report documenting current solar access percentages by month (Correct answer)
- An interconnection agreement with the utility
- A string sizing calculation for future reference
Correct answer: A baseline shading analysis report documenting current solar access percentages by month
Documenting current shading conditions creates a legal and technical baseline to demonstrate future shading impact if the trees grow and reduce energy production, supporting any dispute or easement claims.
Question 4: In NREL's PVWatts calculator, the 'Array Type' input that accounts for single-axis tracking of the modules affects which output parameter most directly?
- DC system size in kW
- Annual AC energy production (kWh) (Correct answer)
- Module efficiency temperature coefficient
- Inverter efficiency curve
Correct answer: Annual AC energy production (kWh)
Selecting single-axis tracking in PVWatts increases the modeled annual AC energy production because the array follows the sun's east-west daily motion, capturing more irradiance throughout the day.
Question 5: A ground-mount array is designed with row spacing to prevent self-shading at a specific 'design sun altitude.' Which sun altitude angle is the NABCEP standard practice for limiting inter-row shading in the continental US?
- Solar altitude at summer solstice solar noon
- Solar altitude at 10 AM on the winter solstice (approximately the lowest critical sun angle) (Correct answer)
- Solar altitude at the spring equinox at 3 PM
- Solar altitude at 6 AM on the fall equinox
Correct answer: Solar altitude at 10 AM on the winter solstice (approximately the lowest critical sun angle)
NABCEP best practice uses the winter solstice at 10 AM (and 2 PM) as the design sun angle for row spacing to prevent self-shading during the period of lowest sun angles in the year.
Question 6: What is the primary limitation of using only GHI (Global Horizontal Irradiance) data from a nearby weather station for a tilted array simulation?
- GHI data is only collected during summer months in NSRDB datasets
- GHI must be decomposed into direct and diffuse components and transposed to the plane of array, introducing modeling uncertainty (Correct answer)
- GHI data does not account for ambient temperature effects on module performance
- GHI measurements are only valid for arrays within 1 mile of the weather station
Correct answer: GHI must be decomposed into direct and diffuse components and transposed to the plane of array, introducing modeling uncertainty
GHI must be mathematically decomposed into beam (DNI) and diffuse components using a model (e.g., Erbs), then transposed to the tilted plane of the array (POA), each step adding potential modeling error.
Question 7: A PV system simulation in SAM (System Advisor Model) shows a 'self-shading loss' of 3.1%. This loss specifically represents:
- Energy lost when modules overheat due to reduced airflow from adjacent rows
- Energy lost when one row of modules casts shadows onto an adjacent row at low sun angles (Correct answer)
- Voltage mismatch between differently oriented strings
- Energy loss due to module soiling from wind-blown debris
Correct answer: Energy lost when one row of modules casts shadows onto an adjacent row at low sun angles
Self-shading (or inter-row shading) loss in SAM represents the reduction in energy output caused by the front rows of a multi-row ground-mount or flat-roof array casting shadows onto the rows behind them.
NABCEP Solar Site and Shading Analysis 4
When measuring roof azimuth with a compass for solar site assessment, which correction must be applied to obtain the true azimuth?