The article details the data obtained from monitoring the photovoltaic (PV) system in 2021-2023, equipped with a module for measuring basic weather parameters. The PV system under consideration, with a peak output of 3.2 kWp, is connected to the electricity grid and is mounted on the flat roof of the building. The annual relative yields of the generated energy were about 10% less than estimated, and their monthly distributions were asymmetric. In the "summer" months (i.e., from April to September), the PV system generated, on average, about 75% of the whole year’s energy. The histograms of the PV system's active power output showed that, on average, about 40% of the time, the PV inverter operated in underloaded mode. For selected weather conditions measured on site, a regression analysis was conducted with the active output power of the PV system. The main objective of the work was to develop a comprehensive method for analysing PV plant monitoring data and the impact of weather conditions on its performance. The proposed method was realized as a case study for central Poland but can be implemented anywhere.