Irradiance measurements to calculate the Performance Ratio (PR) for solar parks sometimes cause controversy: A few years ago, I visited an O&M conference, where cheating with irradiance measurements was openly discussed on the podium. There were about 200 attendees in the room; And after a presentation on irradiance measurement at a solar conference, our R&D manager was asked whether Kipp & Zonen could make instruments that were less (!) accurate.
This shows that irradiance measurements can cause headaches; but they are absolutely necessary for calculating the Performance Ratio. Besides the output generated relative to the solar irradiance received on the panels, PR is the figure looked at by most players involved in the PV power plant business.