The heterogenous nature and spectral distribution of rear plane-of-array irradiance RPOA presents challenges when measured by small-area sensors such as pyranometers. Bifacial reference modules serving as large-area sensors can simplify irradiance monitoring because their electrical response follows that of the power generating modules in an array. This article compares RPOA and effective irradiance GE measured by calibrated reference modules against three commonly used small-area sensors including pyranometers, reference cells, and photodiodes. A technology-matched monofacial module is mounted side-by-side with the bifacial reference to decouple effective irradiance measurements into front and backside contributions. The results show that RPOA and GE measurements made with reference panels have the best correlation to reference cells. The mean absolute errors between the two measurement approaches are 9% relative, 4 W/m2 absolute for RPOA and 4% relative, 7 W/m2 absolute for GE. When GE measurements from the four sensor types are used to predict string-level power, the reference panel measurements show a 3.4% prediction error, which is comparable to that achieved when using GE measurements from pyranometers (3.0%) and reference cells (2.9%) thereby suggesting that reference modules can be used to accurately measure RPOA and GE in bifacial systems.