When people speak of zero-emissions vehicles, they are really referring to tailpipe emissions. Pure electric vehicles produce no emissions locally, which make them ideal for use in urban environments. But what about upstream emissions from power plants, especially those that still burn coal? Does the demand to power all those electric vehicles with a dirty fuel source change the GHG calculation? It sure does.

In 2014, Stanford University student David Heinz published a paper showing the relative CO2 output for a variety of gasoline and electric vehicles taking into account well-to-wheel (extraction of petroleum, refining, and transportation, etc.) CO2 sources as well as vehicle efficiency. He found that in states where a larger percentage of electric energy is produced from so-called dirty sources like coal, the actual all-in vehicle CO2 emissions can be even higher than a gasoline-powered vehicle.

Heinz showed, for example, that a Honda Fit with an EPA fuel economy rating of 36 mpg and an energy consumption rating of 3.357 megajoules/mile produced 268 grams of CO2 per mile. In contrast, an all-electric BMW i3 with an energy consumption rating of 0.9720 megajoules/mile produced 20.3 grams of CO2 per mile in Washington state, where the electric grid is considered to be very clean because of its vast hydro-electric resources. The CO2 output in California, which in 2014 was considered close to the national average for grid cleanliness, was 98.7 gm/mile. In Indiana, however, which draws about 80% of electric needs from coal, the BMW i3’s CO2 output was 313 gm/mile. That’s about 45 gm/mile more CO2 than the gasoline-powered Honda Fit.

Unfortunately, not much of this type of data exists for commercial vehicles. Transit buses, however, have been extensively studied, and they could probably be lumped into the Class 7 vehicle category for comparison purposes. According to Jimmy O’Dea, a vehicles analyst with the Clean Vehicles Program of the Union of Concerned Scientists, electric transit buses compare very favorably against their diesel and natural-gas-powered counterparts.

“Your typical transit bus gets about 3-4 mpg on diesel,” O’Dea told HDT. “An electric bus gets about 18 mpge. That’s about a four-fold improvement in efficiency from the electric drive system.”

That’s just the vehicle side. It doesn’t include the upstream part.

“In California [because of its relatively clean grid], an electric bus has 70% lower GHG emissions than a diesel or natural-gas-powered bus,” he says. “The same would apply in areas of the country, like Washington, upstate New York, New England and places with very clean grids.”

Overall, O’Dea says that for reasons that have to do with mechanical efficiency, an electric passenger vehicle will be about three times more efficient than a gasoline-powered car, but a heavy-duty electric vehicle is more like four to five times more efficient.

Determining the well-to-wheel CO2 footprint of a heavy vehicle is very complicated, but it would surely follow that the emissions in the “dirtier” states would not be as compelling as those in the states with cleaner sources of electricity.

O’Dea says that with more and more coal plants closing and capacity increasing from solar and wind sources, the CO2 picture is only going to continue improving.