The nominal efficiency of PV-modules is around 12%. The overall efficiency of PV systems lies typically around 8-9%. The purpose of this work is to understand and explain such a difference and, consequently, identify actions which can be significant in the design and the optimization of PV systems. We have monitored in great detail, two PV systems located in the city of Geneva. One of 7.5 kW is connected through three inverters to the three phases of the grid. The other one of 2.2 kW is directly connected to trolleybus lines at a DC voltage of 600 V. All losses involved in such systems depend on local conditions and are closely correlated. It may be difficult to evaluate each one separately. Nevertheless, by appropriate selections of data and by testing components in separate experiments, we have evaluated the different losses affecting the performances of the systems under study. These are distributed in several contributions of comparable significance; for example, in the former system: PV-modules characteristics lower than expected, MPPT efficiency decrease with solar radiation, ohmic losses in wiring, temperature effects, incidence angle effects through glass, poor inverter efficiency during operation, inverter startup near threshold, little shadowing due to a security fence. Spectral effects arc significant, but not quantifiable in this frame. Altogether, the nominal efficiency of 12.4% is reduced to 7.8%, i.e. an overall loss factor of 37% over the rated energy.