Nearly half of all the battery-electric vehicles and plug-in hybrid electric vehicles registered in the U.S. in 2013 came in California, according to the U.S. Energy Information Administration (EIA).

Total U.S. sales of plug-in electric vehicles (PEVs) have increased in recent years, but still represent only about 0.7 percent of new vehicle sales in 2014 so far, up from 0.6 percent in 2013 and 0.4 percent in 2012, the EIA reported. There were about 70,000 EVs and 104,000 plug-in hybrids (PHEVs) registered in the U.S. in 2013.

Several states offer tax incentives to reduce the upfront cost of PEVs to consumers. These incentives are in addition to a federal (nationwide) tax credit, which ranges from $2,500 to $7,500 depending on battery capacity and gross vehicle weight. For instance, California offers rebates of up to $2,500 for EVs that run only on a charge, and $1,500 for PHEVs, which can also run on gasoline.

California implemented a zero-emissions vehicle (ZEV) mandate that requires automobile companies to produce for sale a certain percentage of zero emission vehicles, such as electric and hydrogen fuel cell. By 2025, approximately 15 percent of all new light-duty vehicles sold in the state must be either electric or fuel-cell powered.

Nine states have agreed to follow California's ZEV mandate: Connecticut, Maine, Maryland, Massachusetts, New Jersey, New York, Oregon, Rhode Island, and Vermont. These 10 states represent close to one-quarter of the U.S. light-duty vehicle market, according to the EIA.