In order to simulate historical Global Horizontal Irradiance (GHI) and Direct Normal Irradiance (DNI) solar resources, a method using MODIS level 3 (L3) daily satellite data as input to the REST2 clear sky model is presented to derive clear sky solar irradiance for California. MODIS L3 precipitable water (PW) and especially aerosol optical depth (AOD) were found to be significantly biased and were therefore calibrated based on AOD and PW from Aerosol Robotic Network (AERONET) ground monitoring sites. For reference, MODIS input data was replaced by the following input data sources: 3 hourly PW and AOD from Monitoring Atmosphere Composition and Climate (MACC) and monthly climatological Linke Turbidity from Solar radiation Data (SoDa). Similarly, other clear sky models, specifically Ineichen and McClear, were also run for reference. Validation was conducted using irradiance anomalies defined as the difference between irradiance and its 15 day moving average against ground measurement from California Irrigation Management Information System (CIMIS), National Renewable Energy Laboratory (NREL), and Integrated Surface Irradiance Study (ISIS) stations. It was found that the calibration of MODIS data markedly improves the accuracy of modeled Gill and DNI anomalies and REST2 clear sky model with calibrated MODIS data achieved the highest accuracy among all model and input data combinations. The improvement in accuracy of PW and AOD input data through calibration is relatively more important than the choice of clear sky model. (c) 2015 Elsevier Ltd. All rights reserved.