A computer-implemented system and method for correlating overhead sky clearness for use in photovoltaic fleet output estimation is provided. A temporal distance that includes a physical distance between two locations, which are each within a geographic region suitable for operation of a photovoltaic fleet, is determined in proportion to cloud speed within the geographic region. A set of input sky clearness indexes is generated as a ratio of each irradiance observation in a set of irradiance observations that has been regularly measured for one of the locations within the geographic region, and clear sky irradiance. A clearness index correlation coefficient between the two locations is determined as an empirically-derived function of the temporal distance. The set of input sky clearness indexes is weighted by the clearness index correlation coefficient to form a set of output sky clearness indexes, which indicates the sky clearness for the other of the locations.