A grid connected single-phase residential photovoltaic system is modeled and simulated using a detailed transformer model. The transformer model takes into account the non-linearity of the core material, which further affects the harmonic currents injected into the utility grid. Furthermore, suitable models for the photovoltaic array and the inverter have been used to assemble a system model in the field of standard form state equations. Comparison with published results has been made with a prototype installation of the same configuration. Very good agreement is accomplished between the predicted and measured values. Thus, a valid powerful tool is given for studies of such grid connected single-phase photovoltaic systems.