1. A computer-implemented method for efficiently performing area-to-point conversion of satellite imagery for photovoltaic power generation fleet output estimation, comprising: accessing satellite imagery data of overhead sky clearness for a geographic region suitable for operation of a photovoltaic station; identifying a set of pixels within the satellite imagery data corresponding to a bounded area within the geographic region, each pixel representing collective irradiance over a plurality of points within the bounded area; setting an area clearness index for the bounded area as an average of point clearness indexes derived from the collective irradiance for each point; expressing a variance of the area clearness index as an average of the variance of the point clearness indexes; proportioning the variance of the area clearness index to an area metric corresponding to the physical area covered by each pixel; and determining a variance of the point clearness index for one such point as a ratio of the variance of the area clearness index and the physical area covered by each pixel given the point clearness index for the point, cloud speed, and time interval, wherein the steps are performed on a suitably-programmed computer.