From Energy Independence for North America –  Transition to the Hydrogen Economy, U.S. Department of Energy:

The long term vision to achieve energy independence is the transition to a hydrogen economy.  However, hydrogen is not a true energy source, it is an “energy carrier” similar to a battery where energy can be stored. Hydrogen will need to be extracted from hydrocarbon fuels or from the electrolysis of water. Both of these processes require the expenditure (input) of energy to produce pure hydrogen.

How much hydrogen is needed by the transportation sector? The EIA estimates that by 2025, about 13 million barrels of oil equivalent per day would be needed to supply the energy needs of all light-duty gasoline highway vehicles (cars and light trucks). Assuming efficiencies in the hydrogen production chain as reported in literature [18] and hydrogen fuel cell vehicles achieving double the fuel efficiency of gasoline internal combustion engines, 647 thousand tons of hydrogen per day would be needed.

The next question is how this huge quantity of hydrogen will be produced and how much will it cost. Assuming that hydrogen will be produced by electrolysis of water only, it is estimated that about 1,300 GW of electricity would be required to produce enough hydrogen to fuel all of the light-duty vehicles. This would require the generating capacity of nearly 1,500 additional 1GW nuclear power plants operating at an 88 percent capacity factor or 3.7 million 1MW wind turbines at a 35 percent capacity factor, or a combination of these and other alternatives such as hydrogen from biomass and solar generated electricity. This is a tremendous amount of additional electric generating capacity which would need to be put in place for a hydrogen economy to materialize just for light-duty vehicles.