NOVELTY - The predicted global horizontal irradiance for the location of the photovoltaic plant is determined at a future time from the meteorological data. The array data is accessed (320) for the photovoltaic array of the photovoltaic plant. A predicted plane of array irradiance is calculated for the photovoltaic array at the future time based upon the predicted global horizontal irradiance and the array data. A power output of the photovoltaic plant at the future time is forecast (330) by a computing system having a processor and a memory, based on the predicted plane of array irradiance.    USE - Method of forecasting power output of photovoltaic plant for controlling energy storage device.    ADVANTAGE - A forecast can allow for operations to mitigate effects of a forecasted shortfall in solar power output at, for example, the local level, where the mitigation operations can include directing an energy management system to shed noncritical loads and/or ramping down the power produced by the photovoltaic plants at a rate that is acceptable to the utility to which the photovoltaic plants provide power. A solar power forecast system can provide more certainty as to the amounts of power that the photovoltaic plant can produce in the future, which can benefit the utility by allowing utility personnel to better plan how to provide power to various loads, such as by contracting for the delivery of power in advance so as to avoid having to purchase power on the spot market. Since the power output of the photovoltaic plant is predicted to decrease, the shedding of loads through an energy management system allows a load to correspondingly decrease. The decrease in load can effectively cancel out the decrease in photovoltaic plant power output. Thus, the utility can generally not be affected by the decrease in photovoltaic plant power output. The forecasted photovoltaic plant output can be compared with the actual PV plant output to see if the actual PV plant output is much less than the forecasted PV plant output. Thus, allowing a photovoltaic plant operator to determine whether there is a problem with the PV plant that is reducing the output so as to lead to improvements in the economic viability of the photovoltaic plant for the operator. An energy storage device controller can assist the photovoltaic plant in providing the committed-to power to the utility. The energy storage device controller can transfer energy to the energy storage devices after the photovoltaic plant output shortfall concludes.    DETAILED DESCRIPTION - INDEPENDENT CLAIMS are included for the following:    (1) computing system for forecasting solar power output of photovoltaic plant;    (2) computer-readable storage medium storing program for forecasting solar power output of photovoltaic plant;    (3) computing system for predicting decrease in solar power output of photovoltaic plant;    (4) solar power inverter;    (5) method of controlling power produced by one or more photovoltaic modules; and    (6) method of controlling energy storage device.    DESCRIPTION OF DRAWING(S) - The drawing shows a flowchart illustrating process for forecasting solar power output.    Solar power output forecasting process (300)    Step for receiving meteorological data (305)    Step for obtaining meteorological data specific to photovoltaic plant (315)    Step for accessing array data for photovoltaic plant (320)    Step for forecasting power output of photovoltaic plant (330)