The systematic monitoring of plant parameters and the comparison of monthly yields is an essential basis for the evaluation of every PVS as well as for technical improvements for future concepts and constructions. Since few PVS in Saxony have the capacity for remote readouts, obtaining any testable data often depends mainly on the persistence and correctness of the owner of the plant.

The following results and discussions are mainly based on measurements of the PV energy fed into the grid of the regional energy suppliers. Here it is important to consider whether and how the connection to the net was established. Some PVS from the 1000- roofs-program feed also today only the excess energy to the grid (as originally requested in this program), whereas all new PVS feed all the generated energy into the grid. Usually one time per year the generated energy is registered by the energy supplier, but some plant operators register the monthly yields also.

The fundamental basis of the evaluation of the PVS results is the solar irradiation at the location, which can, however, be influenced considerably through local factors (shading, generator orientation, etc.).

The DWD irradiation measurements in Dresden were proved to be representative for Saxony [3]. In Figure 2 the monthly course of the mean value of the irradiation in Dresden is shown together with the data for the years 2002 and 2003. It is evident that in the year 2002 irradiation corresponded exactly the annual average (meteorological regular year). By contrast, this value was exceeded in the year 2003 ("century summer”) by 15%.

To compare the capacity of PV-plants, generally the normalized output power (yield, measured in kWh/kW) is used. The performance ratio PR better characterizes the design and the component properties of a PVS, but the needed irradiation measurement in the module plane is only at few PVS available.

Figure 3 illustrates the classified yields (as far as they are accurate by the data basis) of about 170 PVSs in Saxony in the years 2002 and 2003.

The depicted PV-systems clearly reflect the changed conditions of irradiation in both years. The average yield in 2003 is about 15% higher than the yield of 2002. About 30 PVS reached a yield of more than 1000 kWh/kW in 2003 (compared with zero systems in 2002). The highest yield was 1105 kWh/kW. Nevertheless, the figure also shows a number of PVS with unacceptable low yields.

Figure 4: Monthly yield in 2002 for about 60 PVS depending on the commissioning date

Different influences on the monthly and thereby also annual yields are known, and it is difficult to select any from individual output measurements. From the experiences with PVS in Saxony, the following is noteworthy: [16]

be reached. In [5] and [6], about the same amount of yield increase depending on the year of construction was reported, without known or detectable selective influencing factors.