1. A computer-implemented method for estimating power data for a photovoltaic power generation fleet, comprising: assembling sets of solar irradiance data for a plurality of locations representative of a geographic region within which a photovoltaic fleet is located, 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; converting the solar irradiance data in the time series over each of the time periods into a set of clearness indexes relative to clear sky global horizontal irradiance and interpreting the set of clearness indexes as irradiance statistics; combining the irradiance statistics for each of the locations into fleet irradiance statistics applicable over the geographic region; building power statistics for the photovoltaic fleet as a function of the fleet irradiance statistics and a power rating of the photovoltaic fleet; and generating a time series of the power statistics for the photovoltaic fleet by applying a time lag correlation coefficient for an output time interval to the power statistics over each of the input time intervals.