Photovoltaic generation (PV) systems are growing as one of renewable energy resources in the world from their merits of low greenhouse effect gas exhaustion and less fossil fuel consumption. However, PV output widely varies depending on the insolation condition and is uncontrollable. When huge amount of PV systems are interconnected to a grid, the supply and demand balancing, one of the most important and fundamental power system operations, might become more difficult. In such situation, PV output forecast should be taken in to account in the unit commitment process and/or the online balancing operation. Particularly, the generation reserves should be set aside so that the PV output fluctuation and PV output forecast error can be compensated. From this viewpoint, this paper proposes an operational cost evaluation method considering the probabilistic feature of forecast error. Furthermore, a method for estimating the adequate reserve required for large PV installations is presented.