The fuel cell electric vehicle (FCEV) is a promising transportation technology for resolving the air pollution and climate change issues in the United States. However, a large-scale penetration of FCEVs would require a sustained supply of hydrogen which does not exist now. Water electrolysis can produce hydrogen reliably and sustainably if the electricity grid is clean, but the impacts of FCEVs on the electricity grid are unknown. In this paper, we develop a comprehensive framework to model FCEV-driving and -refueling behaviors, the water electrolysis process, and electricity grid operation. We chose the Western Electricity Coordinating Council (WECC) region for this case study. We modeled the existing WECC electricity grids and accounted for the additional electricity loads from FCEVs using a Production Cost Model (PCM). Additionally, the hydrogen need for five million FCEVs leads to a 3% increase in electricity load for WECC. Our results show that an inflexible hydrogen-producing process leads to a 1.55% increase to the average cost of electricity, while a flexible scenario leads to only a 0.9% increase. On the other hand, oversized electrolyzers could take advantage of cheaper electricity generation opportunities, thus lowering total system costs.