I think people would consider an EV if it gives a minimum range of at least 50 miles before charging. According to a survey done by the U.S. Bureau of Transportation Statistics, it was found that 75% of Americans travel less than 40 miles a day. Based on this figure, an EV having a range of more than 40 miles will be successful. The Volt (40 miles) and Nissan Leaf (100 miles) have achieved this limit.

Source:

http://www.chevrolet.com/pages/open/default/future/volt.do