Electric Cars Don't Need More Generating Capacity?

From a Technology Review piece skeptical of Elon Musk's ambitions:

A 2007 study by the Pacific Northwest National Laboratory found that without adding a new plant or transmission line, the U.S. grid could reliably charge 84 percent of the nation’s cars, pickups, and SUVs.

Without reading the study I understand the logic: lots of 24-hour generating capacity goes unused at night.  The cost would be for the fuel, coal or natural gas, to run it, but not the capital expense of building new generators.  (Though a 10-year old study might be somewhat out of date.)