The number of EVs on U.S. roads will double by 2021, according to an association representing privately held electric companies.

In "Electric Vehicle Sales Forecast and the Charging Infrastructure Required Through 2030," authors Adam Cooper and Kellen Schefter of the Edison Electric Institute predict the sales of 1 million EVs in the next three years, matching the number sold over the past eight.

"Automakers are responding to customer demand and are developing more EV models, including both plug-in hybrid electric vehicles (PHEVs) and battery electric vehicles (BEVs), that are increasingly cost-competitive with internal combustion engines," the authors wrote, in part. "In addition, customers are purchasing EVs in record numbers, and electric companies are working with stakeholders to move the EV infrastructure market forward."

By 2030, the report predicts:

• A total rolling stock of 18.7 million EVs, representing about 7% of all U.S. vehicles.

• Average annual sales exceeding 3.5 million EVs, representing about 20% of all new-vehicle sales.

• The need for an EV charging infrastructure that includes 9.6 million ports nationwide.

"Electric company participation in the development of EV charging infrastructure supports state-level clean energy and transportation goals, expands customer choice, and helps to scale and ensure the availability of needed EV charging infrastructure to support the growing number of EVs on U.S. roads," the report concluded.