The integration of on-grid rooftop solar power plants (PLTS) in the industrial sector can increase energy efficiency and reduce carbon emissions; however, it also introduces challenges related to harmonic distortion affecting power quality. This study analyzes the impact of connecting a 790 kVA inverter-based rooftop solar system (accounting for 14.2% of contracted power) on harmonic distortion at the LKK Feeder, KHX Substation, in Bandung, by measuring before and after integration using a Power Quality Analyzer. The results show that Total Harmonic Distortion of Voltage (THDv) increased from 1.74–2.19% to 2.06–2.66%, remaining below the threshold of <5%. Meanwhile, Total Harmonic Distortion of Current (THDi) rose significantly from 6.68–8.94% to 13.27–26.83%, exceeding the <5% limit, especially during operational hours and on phase S. Dominant harmonics were found at the 5th and 7th orders, mainly originating from the PLTS inverter and non-linear industrial equipment. Over 3,500 incidents of Rapid Voltage Change and flicker spikes exceeding standards were recorded. It is recommended to install harmonic filters, balance loads, monitor power quality in real-time, and perform routine infrastructure inspections. This study serves as an important reference for the implementation of on-grid rooftop solar systems to achieve energy efficiency without compromising the reliability of the power system.