I. Introduction
The two most popular tools used by the photovoltaics (PV) industry for measuring irradiance are thermopile pyranometers and calibrated PV Reference Devices (PVRDs). Currently, the PV community has a poor understanding of which device is better suited for PV applications. In this work, we explain why PVRDs are better suited for PV applications, and we calculate typical measurement uncertainties for measurements made with both PVRDs and pyranometers.