This paper proposes a system composed of Hybrid Photovoltaic, Wind Power Generation and Storage Bank(HPWS), in which Storage System is controlled for smoothing the output power. An optimization strategy of smoothing control based on Low-pass fliter is proposed in this paper. A determination of power capacity of storage system which is sufficient for the smoothing control but as small as possible is very important. An evaluation method for the determination of the power density and power capacity is presented. Smoothness function and changing times of storage system in a day is caculated to evaluate the effect of smoothing control stratey.