The battery energy storage system (BESS) is a popularly used source to smooth the power fluctuation caused by variation of wind speed and solar irradiance. The forecasted data of wind and solar irradiance may contain bias error and does not provide the true power available from wind and photovoltaic generating systems. This study proposes a method based on discrete Kalman filter (DKF) to eliminate the bias error present in the forecasted data so that true power of wind and photovoltaic systems could be predicted. A DKF-based state-space approach is utilised to predict the true power obtained through wind and photovoltaic generating systems. The proposed approach also attenuates the power fluctuation of wind and PVGSs considering the desired power dispatched to the load as well as evaluates the BESS power needed for this purpose.