Created by E.Ö., SUPSI, on 27.11.2025
Shading on photovoltaic (PV) modules directly reduces the electrical output of the module by reducing the amount of sunlight reaching the solar cells. This can be caused by a variety of objects such as trees, buildings, snow accumulation, soiling or even self-shading (Figure 1 and Figure 2). When a cell is shaded, it produces less current, which can reduce the output of the entire cell array. So even partial shading on a small section of the module can significantly affect performance due to the series connection of the solar cells. In some cases, shading can cause heat dissipation, which can compromise the reliability of the module (Figure 3). Partial shading can negatively affect the performance, durability, and long-term reliability of PV modules.
Figure 1: (left) Row-to-row shading [1]. (right) Self-shading from Helioplant solar trees in Sölden Austria [2].
Figure 2: Shading due to (left) snow and (right) soiling [3].
Figure 3: Partial shading on a cell causing hot spot.
Localized Heating and Hot spots
When a portion of a PV module is shaded, depending on the system’s electrical operating point, heat dissipation may occur on the shaded cells, leading to localized heating. This phenomenon, referred to as «hot spots» or «hot cells,» can cause long-term damage by degrading the module’s materials, such as the encapsulant, backsheet, or glass, and may even result in the permanent failure of individual cells or the entire module (Figure 4 and Figure 5). Over time, this localized heating can accelerate material degradation, reduce the module’s lifespan, and increase maintenance costs.
Figure 4: Discolouration of (left) encapsulant and (right) backsheet of the module after 13 months of operation with a shadow mask. The discolouration of the backsheet is indicated by a red arrow. Dashed lines indicate the edges of the solar cells [4].
Figure 5: Fire incident in PV array initiated by hot spot failure [5].
Furthermore, frequent shading can exert stress on bypass diodes, which may fail and consequently be unable to protect the cell string as intended. As highlighted in [link to stressors page], the high irradiance levels expected in alpine conditions (exceeding 1400 W/m² [6], and potentially higher depending on orientation) can be particularly critical. Instantaneous irradiances of up to 1800 W/m² have been measured in the Swiss Alps. In cases of row-to-row partial shading, since modules are typically installed at steep angles and the spacing between rows is often limited, repetitive partial shading may occur. If the current generated by the module approaches or exceeds the rated limits of the diodes, their reliability may be compromised. If a bypass diode fails, hot spots with even higher temperatures may develop due to the absence of diode protection.
Mitigation Strategies
- Optimized module placement: Carefully plan the positioning of PV modules to minimize shading from surrounding objects, terrain, or other system components throughout the day and across seasons.
- Selection of diodes with higher current thresholds: Depending on the expected irradiance levels (considering both front and rear sides), diodes with higher current ratings can be selected to improve reliability under high-irradiance conditions.
- Additional diodes per module: Some PV modules now incorporate a greater number of bypass diodes to further enhance performance under partial shading conditions.
- Shadow-tolerant cell designs: Increasingly, PV modules are being developed with solar cells, such as back contacted solar cells, that exhibit partially improved shadow tolerance, making them an option for environments with frequent or repetitive partial shading.
- Use of microinverters or power optimizers: These devices allow each module to operate independently, minimizing the overall impact of shading on system performance. However, the long-term reliability of this additional equipment must also be carefully considered.
References
[1] Y. Sun, X. Li, R. Hong, and H. Shen, “Analysis on the Effect of Shading on the Characteristics of Large-scale on-grid PV System in China” Energy Power Eng, vol. 05, no. 04, pp. 215–218, 2013, doi: 10.4236/epe.2013.54b042.
[2] IEA PVPS Task 13, “Performance of Partially Shaded PV Generators Operated by Optimized Power Electronics” 2024.
[3] IEA PVPS Task 13, “Soiling Losses – Impact on the Performance of Photovoltaic Power Plants” 2022.
[4] E. Özkalay, “Reliability and Long-term Performance of Building Integrated Photovoltaic (BIPV) Modules,” PhD Thesis, 2024.
[5] Z. Wu, Y. Hu, J.X. Wen, F. Zhou, X. Ye, “A review for solar panel fire accident prevention in large-scale PV applications”. IEEE Access, 2020, doi: 10.1109/ACCESS.2020.3010212.
[6] E. Özkalay, H. Quest, A. Gassner, A. Virtuani, G. C. Eder, S. Vorstoffel, C. Buerhop-Lutz, G. Friesen, C. Ballif, M. Burri and C. Bucher, “Three decades, three climates: environmental and material impacts on the long-term reliability of photovoltaic modules,” EES Solar, vol. 1, pp. 580-599, May 30 2025, doi: 10.1039/D4EL00040D.
