Analysis of the Instrumental Drift of our Irradiance Sensors

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(Credits- Image source from Solargris- Global Solar Irradiance of 2023)

Observing The Instrumental Drift of Irradiance Sensors

Irradiance sensors are instruments that measure the intensity of sunlight for a given period of time.

Irradiance sesnsors are essential for monitoring the performance and efficiency of photovoltaic systems(PV systems), which convert solar energy into electric energy (for operation).

Note : They are not immune from instrumental drift caused by aging. These drift has the potential to compromise the measurement accuracy and has an impact on the computation of performance ratio, and index that indicates the relationship between the actual energy yield and the potential theoretical yield of a photovoltaic system.

Soluzione Solare temperature sensor convertors are designed for outdoor use and rapid response to temperature variations (from several milliseconds to a few seconds) in order to guarantee precise Performance Ratio calculations.

  • It is important to periodically calibrate irradiance sensors using an suitable reference sensor,according to the standards that governs the calibration for that specific sensor technology. Calibration (metrological verification) determines the difference between the value measured by sensor and the actual irradiance value. As per ISO 9001 it is prescribed that sensor measuring the irradiance should be recalibrated at least once in every 2 years.
  • We at Soluzione Solare take a great satisfaction in producing irradiance sensors based on photovoltaic cells that are both high quality and reliabe. Every year, some of our respected clients like Suncity (Suncity Srl Roma), who is also in the market of building photovoltaic systems, sends us a small batch of Sunmeter PRO for calibration check. The Sunmeters originated from their most demanding clients, are the ones whose contract include observing the characteristics of irradiance sensor calibration for the best optimal control over the performance of their photovoltaic systems (PV systems).

  • We below share the results of our last 2 calibration batches :

           1. September 2023                                                                                  2. November 2023

  •  Here you can the instrumental drift (in %) on Y axis and the primary irradiance sensors over the respective months (in months) on X axis from their previous   calibration

Instrumental drifts were divided into groups

Group Number Instrumental Drift (%)
Group 0 0 to 0.9%
Group 1 1 to 1.9%
Group 2 2 to 2.9%
Group 3 3 to 3.9%
Group 4 4 to 4.9%.


  • Here are the graphs showing the distribution of sensors in the different groups

Results September 2023

Instrumental Drift of Irradiance Sensors
Instrumental Drift of Irradiance Sensors

Results November 2023

Instrumental Drift of Irradiance Sensors
Instrumental Drift of Irradiance Sensors


From the analysis of the data, we can draw the following conclusions

  • The instrumental drift occurance is not an accident, hence we use a method to construct our sensors with a controlled treatment involving stabilization of their electrical performance.
  • Irradiance sensors calibrated in 2023, 2022, and 2021 shows a good agreement in their instrumental drifts (approximately 0.5% per year), while sensors calibrated in 2020 and 2019 shows a dispersion in instrumental drifts ranging from 0.5% to 4% per year. Considering that many professional operators did not purchase PV cell-based irradiance sensors 13 yeards ago due to their significant instrumental drifts,We put our primary focus in reducing the measurement uncertainity over the years and we discovered results that were considered satisfactory which again made a positive impact on the purchasing decisions.
  • Some irradiance sensors showed positive instrumental drift,we also attribute this “anomaly” to our calibration uncertainty and found this seems to occur in a small number of samples as a reults we were inclined to think that its value does not exceed 1.5%.
  • The trend of structural drifts is about 1.6%, for both September (+3.4%) and November
    (-4.7%) where the values are exteme. Every value obtained from the metrological verification (Calibration) is a valuable material for us to further investigate the instrumental drifts.
  • We have Observed that in most of the cases there is a small drift and in some cases there is a high drift which happens to be larger values than we expected. We at Soluzione Solare are interested in both small instrumental drifts and the larger drift which is neglected as “not in practice trend” , since they encourage us to investigate further into the factors influencing the measurement of irradiance under different conditions.
    We hope you have enjoyed this article and that it has provided useful information about our irradiance sensors.‍


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