A system for tuning a photovoltaic power generation plant forecast with the aid of a digital computer is provided. Global horizontal irradiance (GHI), ambient temperature and wind speed for a photovoltaic power generation plant over a forecast period are obtained. Simulated plane-of-array (POA) irradiance is generated from the GHI and the plant's photovoltaic array configuration as a series of simulated observations. Inaccuracies in GHI conversion are identified and the simulated POA irradiance at each simulated observation is corrected based on the conversion inaccuracies. Simulated module temperature is generated based on the simulated POA irradiance, ambient temperature and wind speed. Simulated power generation over the forecast period is generated based on the simulated POA irradiance, simulated module temperature and the plant's specifications and status. Inaccuracies in photovoltaic power conversion are identified and the simulated power generation at each simulated input power level is corrected based on the power conversion inaccuracies.