This document discusses progress toward developing efficient models for calculating irradiance on the rear side of bifacial solar panels. It describes existing industry models and two new models being developed and validated at NREL - a ray tracing model and a 2D view factor model. Both NREL models were found to accurately predict rear side irradiance ratios and bifacial performance gains compared to measured data from systems at NREL and SolarWorld, with errors less than 2%. Key parameters like albedo, tilt, and row spacing were found to significantly impact rear side irradiance. The view factor model will be integrated into NREL's SAM tool while the ray tracing model will be made available as a standalone tool.