Effects of soiling on PV module and radiometer performance

The question of "how much does the output of PV modules and radiometers decrease due to soiling?" is often asked, but data are rarely available to answer the question. Related questions are a) how effectively does rain clean soiled devices and b) how often should radiometers and modules be cleaned? Answers to these questions are essential to predict PV module performance or the accuracy (uncertainty) of PV module performance data. Results of the module and radiometer soiling data from three independent applications are presented in this paper. Data cover soiling periods from 16 months to five years, and results are consistent for all three applications.