Large error of a day-ahead forecast of aggregated power output of photovoltaic power systems (PVS) can threaten power supply-demand balance in electric power system. When the forecast error is large, the power supply flexibility such as load-frequency control (LFC) resources would be exhausted to maintain supply-demand balance. Such a situation should be taken into account in an impact assessment of a day-ahead forecast error by using UC-EDC simulation model. Based on the availability of several minutes interval data of electricity demand and spatial average irradiance, this study proposes a UC-EDC simulation model including constraints on actually required LFC resources. As a result, because power supply resources can be used as much as possible for supply-demand balancing, electricity supply shortage and surplus are smaller than those in a conventional simulation model including constraints on fixed amount of LFC resources regardless of the real fluctuation levels.