A computer-implemented system and method for determining point-to-point correlation of sky clearness for photovoltaic power generation fleet output estimation is provided. A physical distance between two points is obtained, each point being suitable for operation of a photovoltaic station. A temporal distance that includes the physical distance between the two points in proportion to cloud speed is determined. A correlation between sky clearness over the two points is evaluated as an empirically-derived exponential function of the temporal distance. A set of input clearness indexes for one of the points is correlated 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.