During 1994 a PV array of 72 modules à 45 Wp was installed at the roof the main buildning of Dalarna University, Borlange, Sweden. 32 of theses modules were connected to NEG 1600 inverter, and in 1996 the remaining 40 modules were connected to Solvex 2065. After some years the output of NEG 1600 decreased, and the whole system performance was investigated in detail as a student project. The fragile part was found to be the inverter, and when this inverter finally stop working it was replaced by NT 1800 in 2004. The performance of the three inverter systems are monitored and compared, losses and errors are analyzed and presented. The new inverter NT 1800 shows much superior performance as compared to the old Solvex inverter, an increase of 30% of the annual yield can be obtained. Studies show that inverter designs are improved significantly in recent years. Efficiency at partial loads have increased and more accurate MPPT algorithms are employed.