Operation and maintenance companies, PV plant owners and scientific PV institutes have the challenge to monitor PV installations correctly. When a changed performance ratio is recognized, it is to clarify whether the installation or the sensor is wrong. To be sure that solar irradiation sensors for photovoltaic applications are accurate, a cross-check is applied for four different application cases in the laboratory and in the field. These examples include comparisons between measurement devices like a pyranometer, silicon solar cell irradiation sensors or a photodiode sensor. In the end, guidelines for reliable measurements in the field are provided.