The Measurements and Instrumentation Team within the Distributed Energy Resources Center at the National Renewable Energy Laboratory, NREL, calibrates pyranometers for outdoor testing solar energy conversion systems. The team also supports climate change research programs. These activities led NREL to improve pyranometer calibrations. Low thermal-offset radiometers measuring the sky diffuse component of the reference solar irradiance removes bias errors on the order of 20 Watts per square meter (W/m2) in the calibration reference irradiance. Zenith angle dependent corrections to responsivities of pyranometers removes 15 to 30 W/m2 bias errors from field measurements. Detailed uncertainty analysis of our outdoor calibration process shows a 20% reduction in the uncertainty in the responsivity of pyranometers. These improvements affect photovoltaic module and array performance characterization, assessment of solar resources for design, sizing, and deployment of solar renewable energy systems, and ground-based validation of satellite-derived solar radiation fluxes.