In this work, the performance of a PV installation is assessed. The plant consists of a grid-connected centralized system, where the supplied power is not associated with a particular electricity customer. Operational data from this PV plant, ground-mounted and located in the north of Portugal, are now available for a period of 4 years. The plant is equipped with PV modules (Si amorphous cells), with 60 Wp per module, with a total generating capacity of 124.2 kWp. In this installation, 24 inverters are used. To obtain an accurate prediction of the efficiency and power output, the characteristics of all plant components were introduced in the PVsyst software, together with meteorological data, either those collected at a local meteorological station either those provided by Meteonorm. The results obtained through the simulations and the real output power values were compared. In the simulations, some parameters, like orientation and inclination of the PV modules, were analyzed and recommendations for improving the production of the PV system are given.