A Straight Dope Classic from Cecil's Storehouse of Human Knowledge

Do "green" vehicles really save energy? Is a Prius worse for the environment than a Hummer?

April 4, 2008

Dear Cecil:

In the United States most electricity used in electric cars would probably come from fossil fuels, right? Given all the steps involved, does the loss of energy during electrical generation, transmission, battery charging, and electric motor operation make electric cars an energy-losing proposition?

I read an article stating the Prius has a worse impact on the environment than a Hummer because of the enormous pollution created in making the car’s batteries. True?

Cecil replies:

Good questions, complicated answers, even if we dance past the fine points. Not to waffle, but we’d better consider this an interim report. First some definitions:

  • Hybrid-electric vehicle (HEV): This uses an internal-combustion engine for most of its power, but also has an electric motor run from batteries recharged by the engine. Typically the engine shuts off when the car is stopped. HEVs include the Toyota Prius and the Honda Insight.

  • Plug-in hybrid-electric vehicle (PHEV): An HEV that can charge its batteries by plugging into a charger, permitting all-electric short trips. The forthcoming Chevrolet Volt is such a car.

  • Battery-electric vehicle (BEV): an electric car powered solely by batteries.

All three can charge their batteries using regenerative braking, which recaptures energy otherwise lost as heat when bringing the car to a stop. That’s a big advantage of electrics.

How much energy is lost getting electricity from the power plant to your PHEV or BEV? Plenty. In the U.S. right now, about 70 percent of the energy used to make electricity — more than four million gigawatt-hours — comes from fossil fuels. About 70 percent of that amount is wasted generating the power and transmitting it to your door. Additional energy is lost when charging batteries and running electric motors. Overall, electric cars use fossil fuel at 20 to 25 percent efficiency, but dismal as that sounds, it beats an internal-combustion car, which typically operates at about 15 percent efficiency. An HEV uses around 0.48-0.74 kilowatt-hours per mile, while PHEVs in electric mode and BEVs use 0.18-0.46 kWh per mile. By contrast, a conventional car getting 25 MPG uses 1.35 kWh/mile. To put the issue in more familiar terms, a PHEV or BEV offers fuel economy equivalent to as much as 188 miles per gallon.

Now let’s talk pollution. A huge advantage of PHEV and BEV cars is that their energy can come from renewable sources, such as hydroelectric, wind, or solar. Even if the energy source is fossil fuel, installing state-of-the-art emission controls on a few big power plants is way easier than installing ’em on hundreds of millions of motor vehicles. What’s more, since many electric plants use natural gas, CO2 emissions from power generation are a modest 1.27 pounds of CO2 per kWh — 1.9 pounds per productive kWh once we account for losses during battery charging and so on. Compare that to gasoline, which produces the equivalent of 3.9 pounds of CO2 per productive kWh.

What about the Prius and its allegedly grim environmental impact? The cause of the controversy seems to be a report called "Dust to Dust" by Oregon-based CNW Marketing Research. The report claims a Prius has a higher lifetime energy cost than a Hummer, an assertion cited by George Will in 2007 in his syndicated newspaper column. But the report is ludicrous. It evidently was self-published, lists no authors, quotes no technical literature, never explains its methodology, and contains numerous unsupported and often bizarre assertions. (Sample: a Prius will have a life span of only 109,000 miles whereas an H1 will last for 379,000 miles, apparently the basis for the contention that the Hummer’s per-mile costs are lower.) Only a fool or a commentator with an ax to grind would take such nonsense seriously.

The same can be said of another anti-Prius yarn circulating over the past year. It originated with an article last spring in the Mail on Sunday, a UK paper, blaming the Prius for devastating environmental damage around a plant in Sudbury, Ontario, that processes the nickel used in Prius batteries. The story was grossly wrong; the major damage occurred more than 30 years ago, long before the Prius. Pollution has since been reduced 90 percent and the region reforested. The Mail yanked the piece off its Web site with an apology, but not before the usual knuckleheads picked it up, folded in the "Dust to Dust" drivel, and spread the result everywhere. Rush Limbaugh is said to have read on the air a version of the story that appeared in a student newspaper.

To be clear, I’ve got no vested interest in electric vehicles. Notwithstanding the above, I’m willing to believe that once all the costs and benefits are totted up they’re not much greener than conventional technology. But I also know this: the era of petroleum-fueled vehicles is drawing to a close. Either we find a substitute or we resign ourselves to a slightly updated version of the transportation options available in 1898. (Thanks to my assistant Una for technical consultation.)

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