How Much Do Electric Cars Pollute? Report Says When and Where You Plug In Matters

May 2, 2016

Photo of three EV charging stations located within a parking garage.

A new study suggests reducing fossil fuel use in the electricity sector and encouraging workplace charging are two important factors in achieving the greatest electric vehicle (EV) emissions reduction.

It's well-known that the transportation sector ranks as one of the U.S. leading sources of greenhouse gas (GHG) emissions, closely following behind electricity production. In 2014, vehicles were responsible for producing 26% of the total U.S. GHG emissions, equivalent to producing more than 1.7 million metric tons of CO2. The largest sources of transportation-related GHG emissions include light-duty passenger cars and vans, as well as trucks and sport utility vehicles.

With the aim of reducing GHG emissions associated with the transportation sector, policymakers have begun supporting measures to increase EV adoption while simultaneously raising the question: are EVs really zero-emission? A new report from the National Renewable Energy Laboratory, Emissions Associated with Electric Vehicle Charging: Impact of Electricity Generation Mix, Charging Infrastructure Availability, and Vehicle Type, addresses that question, finding that the potential for emissions reduction depends on when and where drivers charge their vehicles.

The report presents the emissions findings for a variety of light-duty plug-in electric vehicle (PEV) models under four charging scenarios and five electricity grid profiles. The research found that emissions are highly dependent on the percentage of fossil fuels in the electricity mix at the charging location. There are notable differences in emissions between vehicles charged on high-carbon versus low-carbon grids. Despite this, the study indicates that EVs charged on high-carbon grids produce fewer carbon emissions than those produced by conventional vehicles.

The time at which charging occurs also impacts emissions. According to the study, restricting charging to off-peak hours results in higher total emissions for all vehicle types, as compared to other charging scenarios. Alternatively, allowing drivers to charge their EVs anytime and giving them access to workplace charging yields the lowest level of emissions for the majority of electricity grid profiles.

The researchers also investigated differences between vehicle configurations, including a range of EVs and plug-in hybrid electric vehicles (PHEVs), which use both electricity and gasoline. The method included emissions from the miles driven on electricity as well as those driven on gasoline. Notably, although EVs allowed for more miles to be driven using electricity, in many charging scenarios PHEVs produced fewer total emissions because of their fuel efficiency and the range limitations of EVs.

The study suggests that reducing fossil fuel use in the electricity sector and encouraging workplace charging (which allows for more miles to be driven on electricity) are two important factors in achieving the greatest emissions reduction.

The U.S. Department of Energy's Vehicle Technologies Office sponsored this report, as part of the Workplace Charging Challenge initiative. See the full report for additional details.

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