Electric Vehicles Emissions

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When I think about electric vehicles, I think zero emission 'green' transportation. It seems obvious - assuming a purely electric vehicle (not a hybrid) no gas is burned. This is further reinforced by electric vehicles being marketed as zero emissions.

Yet the energy has to come from somewhere, and it comes from an electricity provider. Thus, electric vehicles emissions are an indirect reflection of the emissions of the owner's electricity provider. While electricity providers may vary in their sources of fuel to generate power, a 2011 report shows that coal and natural gas predominate the energy industry. And when it comes to emissions, the EPA estimates the average CO2 emission rate within the United States to be ~ 0.000706 Metric Tons / kWh (kilowatt hour).

Electric Vehicle Emissions

What does this mean for the electric vehicle? The Nissan Leaf mileage has been estimated at 5.4 Miles-per-kWh (or 0.18 kWh/mile), while the Model S Tesla averaged at 330 Wh/mile (or 0.33 kWh/mile).

If we do the math based upon the EPA emissions cited above, on a per-mile basis - using the lesser quoted mileage of the two electric vehicles above (Nissan Leaf) - the emissions estimates work out to 0.000127 metric tons of CO2 per mile, or 127 grams CO2 per mile1.

Gas Vehicle Emissions

How does this value stack up to a gas vehicle? Again, using the EPA estimates as a reference: emissions for a gas engine are roughly 0.00892 metric tons of CO2 per gallon of gasoline. For the sake of argument, lets assume a vehicle of 30 miles / gallon, this leaves us with gas emissions at 0.000297 metric tons / mile, or 297 grams CO2 per mile.

The Comparison

These figures indicate that the electric vehicle may have lower emissions. Yet the quoted mileage of the Tesla (233 grams CO2 per mile) is almost double that of the leaf, approaching the emissions of a 30mpg car. Alternatively, compare these values to a poorly fuel efficient vehicle and the gap widens. Change the city/driving mileage or plug your electric vehicle into a green plug and once again the values change. Should the energy sector steer more towards green energy, the indirect emissions of the electric vehicle will directly reflect this change. Indeed, there are many variables involved when performing the comparison.

I went into this study hoping electric vehicles stood orders of magnitude apart from that of gas. While in the right circumstances electric vehicles can lower emissions, these estimates brought my hopes down to reality, bringing the term 'zero emissions' into perspective.


1How much is 127 grams? A standard Snickers bar is approximately 2oz, or 67 grams - that's the weight of 2 snickers bars per mile.

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