Plug-in hybrid vehicles are certainly the car of the very near future and a core climate solution. And electricity is the only alternative fuel that can lead to energy independence (see here). But I have a long been concerned that General Motors has overdesigned its showcase plug-in hybrid electric vehicle, the Chevy Volt (see here).

Now a major new study by a team of researchers from Carnegie Mellon University, “Impact of battery weight and charging patterns on the economic and environmental benefits of plug-in hybrid vehicles” (see here [PDF]) confirms my basic analysis that plug-ins make sense, but a 40-mile all electric range does not:

We find that when charged frequently, every 20 miles or less, using average U.S. electricity, small-capacity PHEVs are less expensive and release fewer GHGs than hybrid-electric vehicles (HEVs) or conventional vehicles

Large-capacity PHEVs sized for 40 or more miles of electric-only travel are not cost effective in any scenario, although they could minimize GHG emissions for some drivers.

Bloomberg quotes Jeremy Michalek, an engineering professor who led the study: “Forty miles might be a sweet spot for making sure a lot of people get to work without using gasoline, but you’re doing it at a cost that will never be repaid in fuel savings.”

Note that CMU considered a “high gas price” of $6.0 a gallon, which is the equivalent about $200 a barrel, a reasonable high price case for the next decade.

Perhaps the most significant finding for car companies who want to enter the plug-in hybrid business, minimize costs, and frankly crush GM, is something I have thought for a long time — a very short AER can make sense for a large fraction of drivers:

Our results suggest that for urban driving conditions and frequent charges every 10 miles or less, a low-capacity PHEV sized with an AER of about 7 miles would be a robust choice for minimizing gasoline consumption, cost, and greenhouse gas emissions.

Toyota seems to share the view that an AER far below 40 is optimum, as the Bloomberg piece notes:

Toyota also plans tests this year on a plug-in Prius able to go more than 10 miles on a charge.

The final range is likely to be less than half that of the Volt, said Bill Reinert, U.S. national manager for advanced technology for Toyota City, Japan-based Toyota …

“We believe that if you have a smaller battery charged more frequently, you can run on electricity more of the time, then your carbon emissions are going to be lower overall,” Reinert said.

I’m going to include this figure, even though it is a tad opaque, just to keep you off the streets for a few hours puzzling it out (click here for a bigger picture):


Best vehicle choice for minimum fuel consumption, cost, or greenhouse gas emissions as a function of distance driven between charges across sensitivity scenarios.

Michalek is quoted in Green Car Congress piece explaining why smaller is better, at least when it comes to PHEV batteries:

Larger battery packs allow drivers to go longer distances on electric power. But batteries are heavy and expensive. Over a range of scenarios-including fluctuating gas prices, new battery technologies or high taxes on carbon dioxide emissions-plug-ins with small battery packs are economically competitive with ordinary hybrid and conventional vehicles for drivers who charge frequently.

The study, which was accepted this week for publication in a forthcoming issue of the journal Energy Policy, has public policy implications:

The dominance of the small-capacity PHEV over larger-capacity PHEVs across the wide range of scenarios examined in this study suggests that government incentives designed to increase adoption of PHEVs may be best targeted toward adoption of small capacity PHEVs by urban drivers who are able to charge frequently. Because nearly 50% of U.S. passenger vehicle miles are traveled by vehicles driving less than 20 miles per day (Samaras and Meisterling, 2008; US DOT, 2003), there remains significant potential in targeting this subset of drivers.

Once again, I strongly urge General Motors to revisit this issue of range. The company is on its last legs and simply can’t afford to have a major miscue on what will certainly be among its most important new products in the decade of the 2010.

Finally, I think this study is best looked at as describing the optimal plug-in the 2010s. In the 2020s and beyond, as peak oil and desperation about global warming starts to dominate, longer ranges will make more sense. The Volt may, fatally, be 15 years ahead of its time.

UPDATE: Of course, if we build out a lot of charging stations in parking garages, malls, apartment buildings, residences, and so on over the next 15 years, than a considerably shorter range than 40 miles may still be optimal.

This post was created for, a project of the Center for American Progress Action Fund.