1. A computer-implemented method for determining point-to-point correlation of sky clearness for photovoltaic power generation fleet output estimation, comprising: obtaining a physical distance between two points, each point suitable for operation of a photovoltaic station; determining a temporal distance comprising the physical distance between the two points in proportion to cloud speed; evaluating a correlation between sky clearness over the two points as an empirically-derived exponential function of the temporal distance; and correlating a set of input clearness indexes for one of the points into a set of output clearness indexes indicating the sky clearness for the other of the points using a coefficient of the clearness index correlation.