Recently, researchers at Carnegie Mellon University put forth a study with this conclusion: optimally varying the charging speed of plug-in vehicles can cut the electricity cost in half.

Charging a Prius PHEV

Charging a Prius PHEV

The full study won't be released until sometime later this month.  The study, funded by the CMU RenewElec project and the National Science Foundation, will appear in the journal Applied Energy.

For now, we have these few statements on the study from Carnegie Melon University:

"Although plug-in electric vehicles currently make up a small percentage of the vehicle fleet, federal and state subsidies are promoting electric vehicle sales, and they may become a significant portion of the fleet in the future. If owners regularly plug in these electric vehicles when they get home from work, this would add to greater demand, when the most expensive power plants are running."

"Controlled charging can shift loads later in the night when cheaper power plants are again available.  Controlled charging could also help to manage fluctuations from renewable energy sources like wind and solar power, which change their output as the wind changes and as clouds pass by."

"It is already cheaper to charge an electric vehicle than fill up a gasoline vehicle.  But allowing grid operators to control electric vehicle charging speed could reduce these costs further. We see additional savings up to $70 per vehicle each year or even higher for systems that expect new power plant construction and systems with a lot of wind power."

Carnegie Melon University says that additional studies will need to be conducted to fully understand all of the implications of controlling EV charging.

CMU