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; assembling sets of solar irradiance data for the one point, each set of solar irradiance data comprising a time series of solar irradiance observations electronically recorded at successive time periods spaced at input time intervals, each solar irradiance observation comprising measured irradiance; and converting the solar irradiance data in the time series over each of the time periods into the set of input clearness indexes relative to clear sky global horizontal irradiance, wherein the steps are performed on a suitably-programmed computer.