1. A computer-implemented system for bounding accuracy on a forecast of photovoltaic fleet power generation, comprising the steps of: a data storage comprising computer-readable data, comprising: sets of measured irradiance observations for a plurality of locations representative of a geographic region within which a photovoltaic fleet is located, each set of the measured irradiance observations comprising a time series recorded at successive time periods spaced at input time intervals; and sets of forecast irradiance observations for the plurality of locations over the successive time periods, each forecast irradiance observation matching one of the measured irradiance observations for the same location; a computer comprising a processor and memory within which code for execution by the processor is stored, comprising: a forecast statistics module configured to identify error between the sets of forecast irradiance observations and the sets of measured irradiance observations for each of the locations, and to determine a mean and standard deviation of the error for each location and combining the means and standard deviations for all of the locations into a fleet mean and fleet standard deviation of the forecast error for all locations; a sky clearness module configured to generate a set of sky clearness indexes as a ratio of each measured irradiance observation and clear sky irradiance, and to form a time series of the set of the sky clearness indexes for all of the locations within the geographic region; a fleet statistics module configured to determine fleet irradiance statistics for the photovoltaic fleet through statistical evaluation of the time series of the set of the sky clearness indexes, and to generate a time series of power statistics for the photovoltaic fleet as a function of the fleet irradiance statistics and photovoltaic fleet power rating; and a bounding module configured to associate a statistical confidence with each power statistic in the time series derived from the fleet mean and fleet standard deviation.