Within the German 1000 roof PV programme, about 2000 grid connected PV plants (1-5 kW(p)) with a total peak power of 5 MWp were installed on the roofs of single and two family houses. In the Federal State of Lower Saxony, ISFH has been responsible for the technical inspections and the global monitoring of 172 PV plants up until now. The annual final yields range between 430 kWh/(kW(p)*a) and 875 kWh/(kW(p)*a) with a mean value of 680 kWh/(kW(p)*a). Using the annual in-plane irradiation, we determine annual performance ratios in the range 47.5%-81% (mean 66.5%). A procedure to receive standardized performance ratios is introduced using actual peak powers of the PV modules and inverter-specific efficiencies. Typical curves of monthly values of the performance ratio are recorded. PV plants with low final yields are analyzed with respect to operational failures, partly shadowing effects, and poorness in maximum power point adaptation of some inverters. Optimization potentials are also discussed.