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We report an analysis that compares global horizontal irradiance (GHI) estimates from version 3 of the National Solar Radiation Database (NSRDB v3) with surface measurements of GHI at a wide variety of locations over the period spanning from 2005 to 2012The NSRDB v3 es. timate of GHI are derived from the Physical Solar Model (PSM) which employs physics-based models to estimate GHI from measurements of reflected visible and infrared irradiance collected by Geostationary Operational Envonment Satellites (GOES) and several other data sourcesir . Because the ground measurements themselves are uncertain our analysis does not establish the absolute accuracy for GHI. However by examining the comparison for trends and for PSMconsistency across a large number of sites, we may establish a level of confidence in GHI PSMand identify conditions which indicate opportunities to improve PSM. We focus our evaluation on annual and monthly insolation because these quantities directly relate to prediction of energy production from solar power systems. We find that generally, PSMGHI exhibits a bias towards overestimating insolation, on the order of 5% when all sky conditions are considered, and somewhat less (~3%) when only clear sky conditions are considered. The biases persist across multiple years and are evident at many locations. In our opinion the bias originates with and we view as less credible that the bias stems from PSMcalibration drift or soiling of ground instruments. We observe that GHI may significantly underestimate monthly insolationPSM in locations subject to broad snow cover. We found examples of days where PSM GHI apparently misidentified snow cover as clouds, resulting in significant underestimates of GHI during these days and hence leading to substantial understatement of monthly insolation.Analysis of PSM GHI in adjacent pixels shows that the level of agreement between PSM GHI and ground data can vary substantially over distances on the order of 2 km. We conclude that the variance most likely originates from dramatic contrasts in the ground’s appearance over these distances.

Analysis of Global Horizontal Irradiance in Version 3 of the National Solar Radiation Database