Solar modules deployed in industrial areas have the potential to conserve a significant quantity of national grid electricity; nevertheless, the accessibility of industrial contaminants and emissions creates an unusual surrounding environment for industrial PV plants. Thus, evaluating the performance of industrial solar power plants and highlighting any disruptive agent is crucial for their reliable and feasible operation. The current study investigates the deterioration of a 101 kW On-Grid PV system installed in the aluminum industry that has a newly discovered degrading defect, namely the localised partial cell soiling on industrial PV system, which hasn't been seen or identified in classic studies specifically on industrial PV systems operating on ideal conditions. The underperformance of a solar power plant is identified using comparison between PVsyst simulation results and real plant performance metrics. Moreover, assessment analysis is presented by using characterization techniques such as FESEM, EDX, XRD, and FTIR. The results show that the actual yield (57,807 kWh DC and 56,031 kWh AC), array captured losses (12.257 h/day Lc), array system losses (0.57 h/day Ls), and performance ratio (51 %) all differ from the projected yield (89,637 DC and 86,392 kWh AC), array captured losses (6.256 h/day Lc), array system losses (1.067 h/day Ls), and performance ratio (79 %). Additionally, the performance analysis of the strings connected to both inverters demonstrates that the underperformance is caused by the newly originating defect since the PV-defected inverter strings perform worse than the other one. According to the characterization techniques, the industrial residue material of a specific section of the aluminum industry and the material accumulated on specific cells of panels have similar chemical, morphological, and elemental compositions. BaSO4 is found to be the common compound in both materials. Future PV designs, particularly those aimed at the industrial sector, will benefit from the knowledge of the industrial PV system's defect-originating material characteristics and underperformance.