This paper presents a dynamic model and experimental results of a 7.2 kWp photovoltaic (PV) installation located at the Polytechnic University of Valencia (Spain). The modelling of the monocrystalline cells has been realised in TRNSYS and has been validated during an extensive experimental campaign from January 2001 to March 2003, using the data of a fully monitored PV field. The simulation results with TRNSYS provide an accurate prediction of the long-term performance. In addition to the dynamic models, algebraic methods such as the constant fill factor have also been applied. In the design of PV systems, there are several important uncertainties which have to be taken into account, such as the reduction of power with respect to the nominal power under Standard Test Conditions (STC), the choice of the meteorological database, and the models for the calculation of the radiation on tilted surface and of the cell temperature. These aspects are analyzed thoroughly in this paper, as well as the problems inherent to the PV power injection into the grid.