Charging Your Electric Car At Night May Be Bad For The Environment


A new study by Carnegie Mellon University claims that charging an electric car at night may actually be worse for the environment that charging as soon as the owner gets home. How can that be? It depends on where the local utility gets its electricity from.

Charging an electric car from a coal fired plant is bad for the environnent

Jeremy Michalek, a professor of engineering and public policy and mechanical engineering at CMU, and his colleagues modeled the grid that supplies Washington, Philadelphia, Pittsburgh, Cincinnati and Chicago. “We looked at how power plant operations would change in response to electric vehicle charging load, and we modeled emissions from those plants and their downwind air pollution consequences for human health and the environment,” Michalek explained. “We found that charging [an electric car] late at night reduces power generation costs by a quarter to a third, largely by shifting to cheaper coal-fired power plants. But the extra emissions released as a result can cause 50 percent higher costs to human health and the environment.”

The reason is that utility companies use mostly coal fired generating plants to provide the electricity for overnight use. If those coal plants need to ramp up because there is a higher demand for electricity, they spew more pollutants into the atmosphere than they would if they are just idling along.

“As coal is phased out and the grid becomes cleaner, the emissions implications of charging at night will be mitigated,” Michalek says, “and the benefits of late-night charging for the electricity grid may be good reasons to delay charging. For now, if you live in a coal-heavy region like the Chicago, Washington, D.C., Philadelphia area, delaying charging until late at night can cause more harm than good.”

The research highlights once again why some people insist on saying that an electric car has “zero tailpipe emissions” rather than calling it a “zero emissions” car. Of course where the electricity comes from to recharge an electric car will always be an important consideration. In the long run, eliminating coal fired plants is the best way of lowering total carbon emissions, a concept that is generally accepted by all parties except congressional Republicans.


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I have been a car nut since the days when Rob Walker and Henry N. Manney, III graced the pages of Road & Track. Today, I use my trusty Miata for TSD rallies and occasional track days at Lime Rock and Watkins Glen. If it moves on wheels, I'm interested in it. Please follow me on Google + and Twitter.
  • Kyle Field

    This is a highly regional assessment that is ultimately just another data point indicating that we need to get rid of coal as fast as possible.

    • Steve Hanley

      Agreed. Tell it to the Kook Brothers! ; – )

  • curly4

    The use of coal to produce electric should be stopped now and not in some non defined time in the future. Now natural gas should only be use for a little longer than coal. Coal should be eliminated as a fuel by 2030 at the latest and natural gas a little later but by 2045 or maybe delayed to 2050.

  • Mike333

    More Koch Funded Bullshit.
    Nighttime energy is underutilized. That means it’s produced and never sold to anyone. So, it’s wasted as heat. EV’s come on grid and take wasted energy and then use it.

    Secondly, no additional day time capacity ever need be built.

    As a matter of fact, the solar produced during the day, will then LOWER the cost of energy for every customer on the grid.

  • Mike333

    Philadelphia is powered by nuclear.

  • Eco Logical

    The Carnegie Mellon University study is obviously flawed, the coal plants can’t shut down at night because it’d take to long to start them back up the next day so whether we charge at night or not they’re still going to be spewing out that pollution. We need to replace coal with clean energy asap.

    • Jerry3130

      Agree, the study is flawed. They modeled the emission, and downwind pollution, which is totally unnecessary. What they shoud look is how much base load generation is used by EV instead of being wasted.