Long-term measurements are only available from few PVSs. Figures 5 and 6 illustrate the monthly yields for two typical cases.

The PVS Kirnitzschtal consists of two 20 kW PV power plants, equipped with a total of 756 Siemens M55 modules. In each plant, four inverters PVS-5000 are being used in master — slave operation. The generator plane is optimally oriented (37 degree slope, orientation 12 degrees SW). Because of the location of the PVS in the Kirnitzsch valley, shadows appear in the winter months in the morning and evening hours, which lead to a yield loss of approximately 10%. The results shown in figure 5 nonetheless clarify the reliability of the installed grid connected PV plants since 1994. Aside for the defects in inverters that occurred between May and August of 1997, there have been no problems. The highest yield was reached in the high-radiation year of 2003.

The PVS in Zehren, also brought online in 1994, was equipped with 90 modules of model MQ36 (DASA). The generator plane is well oriented (49 degree slope, orientation 15 degrees SE), a SOLWEX 4090 inverter has been used. The first five years of operation show a normal behaviour of monthly yields according to the corresponding radiation levels; annual yields range between 750 and 840 kWh/kW. After 1999 the yield amount clearly decreased, as a result of the breakdown of several modules. Switching out individual modules did not have far-ranging success, since other modules continued to break down.

Only after a comprehensive replacement of all modules (activated guarantee) in July of 2003 did the PVS once again attain normal yield levels as of August 2003.