Substitution of hydrogen for fossil fuels in motor vehicles will require a significant growth of electric energy consumption, especially in high growth rare states such as California. A major concern is the magnitude of the additional electric power capacity necessary to build a large-scale hydrogen fuel industry in California to accommodate the fuel demand of a 30-million vehicle fleet. Electrolysis of water may be the only practical means to produce hydrogen in sufficient quantity without use of fossil fuels for so large a vehicle fleet. A dynamic model was used to estimate the increasing annual electric energy requirement to sustain long-term growth of adequate hydrogen fuel production in two time sequences. The first extrapolates historic data on population, vehicle, and energy statistics from California State agencies to the year 2010 when a fuel-cell engine vehicle industry could begin to expand rapidly. The second time sequence uses the 2010 values as the initial conditions to examine a range of growth scenarios to the year 2050 when a significant fraction of the total California vehicle fleet could be operated with hydrogen fuel. The results of the model calculations show that even with improved energy consumption efficiency of electrolytic production facilities, the additional electric energy demand to sustain replacement growth of hydrogen fuel in the fleet will require installation of additional generating capacity. (C) 2000 International Association for Hydrogen Energy.