This study presents an experimental and simulation‑based performance analysis of a grid‑connected Photovoltaic (PV) system operating under the climatic conditions of Abbasabad, Mazandaran, a humid and forest‑influenced region in northern Iran. Meteorological data were obtained through the Meteonorm database integrated within PVsyst, and the system configuration consisted of three 270‑W polycrystalline silicon modules, a 1.8‑kW IMC inverter, and auxiliary components including a 25‑A charge controller and two 150‑Ah batteries. The PV modules exhibited a short‑circuit current of 9.06 A, an open‑circuit voltage of 37.2 V, and a maximum power point voltage and current of 31.2 V and 8.25 A, respectively. Simulation results indicate that the system experiences array losses of 0.81 kWh/kWp/day and inverter‑related losses of 0.17 kWh/kWp/day. The average useful energy delivered to the grid is 4.67 kWh/kWp/day, yielding an annual Performance Ratio (PR) of approximately 0.827. Seasonal variations in irradiation, humidity, temperature, and wind speed significantly influence system behavior, with the highest PR values observed during winter months due to lower module temperatures and reduced thermal losses. Overall, the findings confirm that despite the region’s high humidity, dense vegetation, and frequent cloud cover, the PV system maintains acceptable performance and can reliably contribute to residential‑scale electricity generation. The results provide a practical reference for PV deployment in similar coastal and humid climates.