There is a considerable amount of confusion on whether electric vehicles (EVs) bring about environmental benefits in the form of lower CO2 emissions when compared to conventional internal combustion engine (ICE) vehicles.

Who should we believe? – Your back of the envelope calculations. There are different ways of calculating the emissions attributable to plug-in EVs and conventional cars. Below, we provide one such way, based on the data used by the US Environmental Protection Agency (EPA) in its May 2010 final rule on light-duty emissions standards and CAFE standards, which applies to the period 2012-2016.

Using this approach, we compare a 2010 new midsize conventional ICE gasoline car against a similarly sized plug-in EV (N.B., not hybrid). The calculations for estimating the total emissions, both upstream (i.e., associated with electricity generation or oil production, refining and transporting) and tailpipe (zero for electric cars) are provided below, based on data from the EPA and Energy Information Administration (EIA).

The parameters necessary to perform this calculation for a conventional ICE gasoline vehicle are: (1) fuel efficiency in miles per gallon, (2) CO2 emitted when producing and transporting  one gallon of gasoline, and (3) CO2 emitted when combusting one gallon of gasoline . By multiplying (1)*(2) and (1)*(3), and adding them up, we find that total CO2 emissions from this type of vehicles are estimated at 352 grams of CO2 per mile, as shown below.

Calculating the emissions associated with a plug-in EV is equivalent to calculating the CO2 emissions from generating the electricity needed to power the car, as these cars have zero tailpipe emissions. A proper calculation of the upstream emissions induced by plug-in EV would require us to know the charging profile of the vehicle fleet (i.e., the proportion of plug-in EVs that are re-charged at every hour of the day), as the CO2 emitted per kWh produced is not the same at peak than at off-peak times.

Nevertheless, absent a more detailed estimation, a back of the envelope calculation is still possible by using the average CO2 emissions from power generation for a given year (1). In addition, as with conventional ICE cars, we need the consumption efficiency of plug-in EVs in Watt hours (Wh) per mile, which needs to be reduced to account for the efficiency losses in the transmission of electricity through the grid and in charging the batteries. Once this is done, we obtain the adjusted consumption efficiency (2). By multiplying (1)*(2), we find that total CO2 emissions from this type of vehicles are estimated at 176 grams of CO2 per mile, as shown below.

Bottom line: while it is true that upstream emissions are higher for plug-in EVs, a simple calculation reveals that total CO2 emissions from a midsize plug-in EV are likely to be about half of those from a conventional ICE gasoline vehicle. As power generation becomes less carbon intensive, the environmental balance will tilt even more in favour of plug-in EVs.

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