It's impossible to convert barrels of oil into raw energy content and then use that number to draw a determination on how much electricity would be used for electric automobile transportation. There are differences in the efficiency of each technology.

For example, Nissan's leaf uses 32kwh to travel an average of 100 miles. A gallon of gas is the rough equivalent to 36kwh....and we know that a gallon of gas in a typical car will only get about 25 miles in combined driving. With that being the case, a leaf gets about 4 times the fuel economy of a typical 25mpg combined car. This is why it gets a 99mpg combined fuel economy rating.

Now, in the United States we use around 142 billion gallons of gasoline a year (2004 statistic). At 36kwh a gallon equivalent this is the equivalent to 5112 billion kwh. Now, since a leaf is around 4x more efficient than the average car, we have to divide that number by 4. Therefore, it would take 1278 billion kwh of electricity a year to completely replace our average 25mpg combined gas cars with 99mpg equivalent leafs.

A typical nuclear power plant averages 12.4 billion kwh a year. That means, if we were just using nuclear power plants, we would need to build just over 100 to completely replace the gasoline that we use (which, we don't even need to do if all we're trying to do is heavily reduce our oil consumption). That doesn't take into consideration any alternate electricity sources (the good), nor does it take into consideration power transmission losses in our grid (the bad).

All in all, it's completely possible to use electric cars to significantly reduce the amount of oil we use. The biggest hurdle is updating our power grid, which is severely out dated and bleeding power at a ridiculous rate. The second big hurdle is battery performance. We need batteries that can charge faster and allow for a greater range before the average person would even consider buying a plug-in electric car. Both of these are slowly being improved (as is the efficiency of regular gasoline vehicles), but it's going to take a while to get there (especially since our government is presently lacking the money to really fund a lot of these projects).