Is there information about what internal models are available in HOMER for the estimation of daily solar radiation series on an inclined plane, based on hourly average data?

From the hourly data of global solar radiation on the horizontal surface, HOMER calculates the hourly global solar radiation on the inclined surface using the HDKR model.  The Help article entitled "How HOMER Calculates the PV Array Power Output" gives a complete description.  To read that article, please look up "pv power" in the index of the Help system.