This section summarizes the results of tests designed to detect common issues in field solar irradiance measurements. If these issues go unnoticed, they can introduce significant bias and increase uncertainty in resource assessment, performance calculations, and other critical analyses.
The tests aim to highlight patterns that typically indicate one or more common problems.
Note: These tests do not always identify the exact root cause of an issue. In many cases, different problems can produce similar patterns and influence each other.
For example:
- A time shift can make data appear similar to that of a tilted pyranometer and may incorrectly place twilight data in nighttime periods.
- Asymmetric shading or soiling can make irradiance data appear shifted or tilted.
As a general rule, high-quality data should pass all tests. Any failed test should be investigated further.
Always seek troubleshooting and investigation if any of the tests fails.
Time Shift
Shifts in timestamps are quite common in field data and can occur for several reasons.
Data acquisition and SCADA systems may have incorrect timezone settings, and system clocks can drift out of synchronization over time. In addition, errors are often introduced during data handling and processing. For example, sampling conventions (start of interval vs. end of interval) may not be consistently applied, and mistakes can occur during data aggregation, resampling, or merging.
This test evaluates whether applying a time shift would make the data appear more physically consistent and plausible.
Note: Other issues may also cause this test to fail. A failed result should always be followed by further investigation.
For more information check: Time Shift Test
Orientation
Pyranometers must be installed with high precision. According to the IEC 61724-1 standard, Class A systems allow a maximum deviation of ±0.5°, and the instrument level should be checked regularly.
In practice, installation errors of 1° or more are quite common and can be difficult to detect after data collection.
This test evaluates whether the measured data is consistent with the expected sensor orientation and whether an alternative orientation would provide a better match.
Note: A failed result does not necessarily mean incorrect installation. Other factors may produce similar patterns, so further investigation is recommended.
For more information check: Sensor Orientation Test
Nighttime Readings
Solar irradiance should be zero at night, and nighttime data is typically excluded from performance analysis. However, it provides valuable diagnostic information.
Reviewing nighttime data can help identify issues that are not visible during daytime operation but still affect measurements.
Common causes of abnormal nighttime readings include:
- Thermal offsets in thermopile pyranometers (often resulting in small negative values)
- Electrical noise or bias due to wiring issues
This test checks whether nighttime irradiance values follow expected patterns. If the test fails, review system settings and installation details such as dome heating and wiring.
For more information check: Nighttime Readings
Shading Impact
Shading from nearby objects, vegetation, or the horizon can lead to systematic underestimation of solar irradiance. Detecting and quantifying this effect is essential for accurate analysis.
Since pyranometers represent site conditions, shading on the sensor can also indicate shading on PV modules.
This test identifies shading patterns and estimates their impact on energy calculations.
Note: The test does not distinguish between expected shading (due to site design or location) and unintended shading (caused by obstacles).
For more information check: Nearby Shading
Final Note
These tests are intended as indicators, not definitive diagnoses.
If any test fails, further troubleshooting and detailed analysis are always recommended.
If you would like to review the overall report structure again, see: Read and Understand Your EKO Q Data Quality Report.