1. A computer-implemented system for correlating satellite imagery for use in photovoltaic fleet output estimation, comprising: a computer comprising a processor and memory within which code for execution by the processor is stored, comprising: a pixel correlation module configured to correlate a set of pixels in satellite imagery data of overhead sky clearness to a bounded area within a geographic region, each pixel representing collective irradiance over a plurality of points within the bounded area, each point suitable for operation of a photovoltaic system comprised in a photovoltaic fleet; an averaging module configured to convert the collective irradiance into a set of point clearness indexes for each of the points within the bounded area relative to clear sky irradiance, and to average the point clearness indexes in the set of point clearness indexes for each of the points within the bounded area into an area clearness index; an area variance module configured to determine a variance of the area clearness index in proportion to a physical area within the bounded area covered by each pixel; and a point variance module configured to determine, for each of the points within the bounded area, a variance of the point clearness index for that point as a ratio of the variance of the area clearness index and the physical area relative to the point clearness index for that point, cloud speed over the geographic region, and a time interval relating to a time resolution of observation for the collective irradiance.