Rapid and large variation of photovoltaic (PV) power may incur frequency variation in a power system with high PV penetration. In such a case, much more reserve capacity of automatic generation control (AGC) for frequency regulation will be needed. In this paper, the energy storage station (ESS) is used to decrease the ramp rate of the total PV power and consequently decrease the AGC reserve capacity requirement of the power grid. However, as an expensive option, the ESS capacity should be optimally determined considering the balance between ESS cost and the equivalent benefit from the reduction of AGC payment. Therefore, an optimal sizing scheme for ESS in a power system with high PV penetration is proposed to strike a balance between the ESS cost and the reduction of the AGC payment. Firstly, a control scheme of ESS is proposed for smoothing PV power variation, during which the ramp rate of PV power can be finitely controlled with a size-limited ESS. Afterwards, an object function aiming to achieve the tradeoff between the investment cost of ESS and the reduction of AGC payment for frequency regulation is presented. Finally, case studies are conducted with actual PV power data to verify the effectiveness of the presented sizing scheme.