1. A computer-implemented method for bounding accuracy on correlated overhead sky clearness for use in photovoltaic fleet output estimation, comprising the steps of: obtaining a physical distance between two locations in a geographic region suitable for operation of a photovoltaic fleet and evaluating a temporal distance as a ratio of the physical distance and cloud speed within the geographic region; retrieving recorded irradiance measures for the one of the locations comprising a time series of irradiance measures recorded at successive times; retrieving forecast irradiance measures for the one location, each forecast irradiance measure corresponding to one of the recorded irradiance measures; bounding statistical error between the recorded irradiance measures and the forecast irradiance measures by evaluating a mean and standard deviation; relating sky clearness over the two locations with a correlation coefficient by solving an empirically-derived exponential function of the temporal distance; generating an input set of forecast clearness indexes as a ratio of each of the forecast irradiance measures for the one location, and clear sky irradiance; and weighting each forecast clearness index by the correlation coefficient to form an output set of forecast clearness indexes for the other location and proportioning the mean and standard deviation to the output set of forecast clearness indexes, wherein the steps are performed on a programmed computer.