John Voelcker’s article ”How Green Is My Plug-In?” focuses on the carbon impact of plug-in hybrid electric vehicles. Their overall effect will depend, of course, on how many are built in the coming years and just what their electric ranges are. IEEE Spectrum ’s David Schneider discussed this issue with Jeremy Michalek, an assistant professor of mechanical engineering and of engineering and public policy at Carnegie Mellon University. Michalek has an upcoming article on the sizing of batteries for such cars in the journal Energy Policy . According to his calculations, plug-in hybrids with big batteries, like the Chevy Volt, may never save consumers any money.

IEEE Spectrum: Tell me a little bit about your research.

Jeremy Michalek: I do work in three areas: The first is systems optimization. The second is design for market systems—that includes things like trying to measure consumer preferences and predict what people are likely to buy in the marketplace. The third area is green design and environmental policy, and that’s where the plug-in work primarily falls. You can have the greenest thing in the world, but if nobody buys it, it makes zero impact.

Spectrum: What sparked your recent study of plug-in hybrid vehicles?

JM: I was looking at other work going on that examined the economic and environmental implications of plug-ins, and I noticed that people made some assumptions I wasn’t sure were valid. One of those was that they would ignore the weight of all the batteries you would have to add to the vehicle.

I wanted to understand whether that added battery weight was important. If you add it all up—especially for a large-capacity plug-in like the Chevy Volt—you really bog down the vehicle. And that can affect efficiency. So even though you get to drive in electric mode for a larger portion of your miles, you get lower efficiency.

Our study showed that moving from a battery pack sized for 7 miles of electric travel to a heavier pack sized for 60 miles of electric travel would increase the amount of electricity needed to propel the vehicle in electric-only mode by about 10 percent and increase the gasoline needed in gas-only mode by about 8 percent. So there’s definitely an effect, but it is not the most significant factor. What turned out to matter much more is the charging pattern.

Most of the other studies of plug-in hybrids analyze what the vehicle does for the median driver. But we wanted to see how differences in driving patterns would influence the economic and environmental implications of such cars.