A computer implemented method for controlling a power plant system comprising a photovoltaic power source, an inverter, a DC-to-DC converter, an energy storage and an energy storage manager, the method comprising: receiving target value for AC power to be supplied by the inverter; receiving weather forecast information, energy storage status information and photovoltaic power source parameters; generating forecasted energy flow information for the photovoltaic power source based on the weather information and the photovoltaic power source parameters; comparing the forecasted energy flow information and the energy storage status information; and adjusting the target value for the AC power based on the comparison.