The use of string inverters in photovoltaic power plant construction has been rapidly increasing due to reduced installation costs and improved system utilization compared to central inverters. However, with the application of string inverters, the length of DC string lines increases, leading to greater line resistance losses and overall system losses. In this study, we measured the degradation in output performance under standard test conditions (STC) as the DC string length increased. The results showed that for a 4㎟ line with a length of 100 m, the resistance was 0.42 Ω, resulting in a 10% loss in output compared to initial performance. This maximum power point tracking (MPPT) loss can be referred to as system loss. Additionally, a module-level testbed was constructed to collect and analyze data. that the analysis confirmed that resistance loss is proportional to the product of the square of the current flowing through the line and the resistance value. It was also observed that systems with higher resistance exhibited greater system losses, as demonstrated in experiments conducted under STC. Furthermore, when applying string inverters, it is essential to establish design criteria regarding line resistance and length. Analysis using data from irradiation levels above 700 W/m revealed significant system losses in systems with a resistance of 2 Ω.