This paper discusses some of the most important Photovoltaic (PV) systems characteristics related to its global efficiency like MPP tracking efficiency, static efficiency, conversion efficiency as well as the Euro efficiency. The losses models of the power semiconductors constituting the Power Electronics (PE) subsystems as well as the models of transformers and DC and AC cables are reviewed. Three topologies are selected, representing the PV market and simulation results are discussed. Given the importance of the global efficiency of an installation for the owner, recorded field results with the same systems are presented. The main conclusion is that the global efficiency of a PV installation depends not only on the PE subsystem, with or without isolation transformer in the low or high-frequency stages, but also on the DC and AC conduction losses, and on the distance of the installation to the coupling point to the electric grid.