An overview on quality assurance measures for large-scale PV power plants is given. Areas discussed include yield assessment, module testing and power rating, on-site system testing and yield monitoring. In addition, we present a procedure for the analysis of module and system long-term stability using performance monitoring data of systems. For PV systems with standard c-Si modules, calculated rates of change in the initial 5-7 years are statistically insignificant. Although this gives confidence about the fairly good long-term stability of c-Si modules, longer monitoring durations may be needed in order to address degradation rates by monitoring data analysis. Concerning PV performances more generally, today's high Performance Ratio is proof that solar PV has matured into both a highly reliable and well-performing technology