One argument for electric vehicles is that they will pollute less, and emit less CO2, than their internal combustion engine counterparts. But that conventional wisdom isn't necessarily true, say three National Burea of Economic Research economists.

Joshua S. Graff Zivin Erin T. Mansur and Matthew Kotchen basically looked at the power plants from which plug-in electric vehicles were likely to get power when charging. EV power sources are important because electricity generation is the primary source of CO2 emissions worldwide and accounts for more than 40% of domestic emissions in the U.S.

The emissions associated with charging PEVS differ by region and time of day, the economists say. The CO2 emissions per mile from driving PEVs are less than those from driving a hybrid car in the western U.S. and Texas. The ecnomists found that in the upper Midwest, however, charging during the recommended hours at night implies that PEVs generate more emissions/mile driven than even the average car currently on the road. Other regions have marginal emission rates that place PEVs somewhere between a hybrid and a comparable economy car. "Underlying many of our results is a fundamental tension between electricity load management and environmental goals, as the hours when electricity is the least expensive to produce tend to be the hours with the greatest emissions," the economists say.

The economists came to their conclusions using data from the EPA, the Energy Information Administration (EIA), and the Federal Energy Regulatory Commission (FERC). Using the two most popular PEVs on the market -- the Chevy Volt and the Nissan Leaf -- the economists summarize the current PEV technology as requiring 0.35 kWh/mile. They then multiplied this number by the emission rates for fossel-fired power to yield the emission rates of PEVs in terms of lbs CO2 /mile if charging used electricity from that particular unit in a given hour. For the purposes of comparison with other vehicles, the economists used the 0.35 kWh/mile as a conversion to report all vehicle emissions in terms of pounds CO2 /kWh.

The average fuel economy of the U.S. fleet of light - duty, gasoline - powered vehicles is 21.7 mpg. Because combusting a gallon of gasoline releases 19.6 lbs CO2, the average light - duty gasoline vehicle emits 0.90 lbs CO2/mile. To make this number comparable with the emission rates of PEVs, the economists multiplied by 1/0.35 miles/kWh to obtain 2.58 lbs CO2/kWh. This number represents the average emissions rate of light - duty gasoline vehicles in the 2009 U.S. fleet. One conclusion is that a PEV will emit less CO2 than the average light - duty vehicle assuming the PEV’s charge comes from a fossil - fired unit that is below the 87th percentile in emissions.

In contrast, and more importantly, say the economists, a PEV could emit more  CO2 if its charge comes from a fossil-fired unit above that percentile in emissions — roughly 13% of all electricity-generating units.

But the economists say that while these numbers illustrate how PEVs might compare with other vehicles in terms of their emissions, the comparisons are potentially misleading for several reasons. For one thing, they do not distinguish among hours of the day, over which there is substantial variation in emissions rates. Also, the numbers assume that the substitute for a PEV is a random draw from the population of all light - duty vehicles.

To address the latter point, the economists made comparisons with vehicles that are more likely to be substitutes for PEVs: an economy car and a hybrid. Using characteristics of the Nissan Leaf, a set of comparable gasoline vehicles is the Toyota Corolla, Honda Civic, Chevy Cruze, and Ford Fiesta. This set has a 2012 fuel economy average of 31 mpg. Converting these units implies an emissions rate of 1.79 lbs CO2/kWh, which leads to the conclusion that approximately 41% of the fossil units that might charge PEVs over any hour have higher emission rates.

Turning to the hybrids, economists considered the leading seller Toyota Prius, which for 2012 has a combined fuel economy rating of 50 mpg, or an emissions rate of 1.13 lbs CO2/kWh. Only 12% of the fossil-fired units over all hours have emission rates lower than this, implying much scope for PEVs to have higher emission rates than hybrid vehicles. In sum, say the economists, these comparisons demonstrate the importance of identifying the marginal power plant for evaluating the environmental implications of PEVs, as well as the choice of substitute vehicles

The full report is available at the NBER site:  http://papers.nber.org/papers/w18462?utm_campaign=dig&utm_medium=email&utm_source=dig