The proposed approach evaluates the effect of integrating a large number of electric vehicles (EVs) on power grid operation and control. The EV fleets could serve as electricity load when drawing energy from the grid and as energy storage (vehicle-to-grid) when delivering energy to the grid. The paper considers two operating modes for EV fleets which are consumer-controlled and grid-controlled. The power grid generation mix represents a multitude of units including thermal, hydro, and wind. The paper considers the impact of EV battery utilization on offsetting the hourly variability of wind generation units in transmission-constrained power grids. The paper considers charging/discharging schedule of EV batteries and consumer driving requirements on the optimal hourly transmission-constrained commitment and dispatch of generation units in the day-ahead scheduling. The hourly solution of the proposed method will minimize the cost of supplying the hourly load while satisfying the temporal constraints of individual components in power grids.