A computer-implemented system and method for bounding accuracy on a forecast of photovoltaic fleet power generation is provided. Measured irradiance observations for a plurality of locations are retrieved. The measured observations include a time series recorded at successive time periods. Forecast irradiance observations are retrieved. Error between the forecast and the measured observations is identified. A mean and standard deviation of the error is determined and combined into a fleet mean and fleet standard deviation. Sky clearness indexes are generated as a ratio of each measured observation and clear sky irradiance. A time series of the sky clearness indexes is formed. Fleet irradiance statistics are determined through statistical evaluation of the sky clearness indexes time series. A time series of power statistics is generated as a function of the fleet irradiance statistics and photovoltaic fleet power rating. A statistical confidence is associated with each power statistic in the time series.