Question of the Month: What are the average greenhouse gas (GHG) emissions from all-electric vehicles (EVs) and plug-in hybrid electric vehicles (PHEVs)? How do these emissions compare to conventional vehicles?
Answer: In addition to reducing petroleum consumption, EVs and PHEVs have the potential to reduce pollutant emissions, resulting in environmental and human health benefits. Vehicle emissions can be characterized as:
Upstream: Emissions from the production of the vehicle fuel (e.g., electricity).
Tailpipe: Emissions produced by a vehicle while operating.
Well-to-wheel: The combination of upstream and tailpipe emissions.
EVs are often referred to as “zero emission vehicles,” meaning they do not emit any tailpipe emissions. Therefore, the well-to-wheel GHG emissions associated with EV operation are entirely dependent on the source that is used to produce the electricity that powers the vehicle. Emissions from electricity production depend on the efficiency of the power plant and the types of fuel sources used. Based on the U.S. average electricity production fuel mix, the GHG emissions are lower for an EV using electricity generated from power plants than a vehicle running on gasoline or diesel. If electricity is generated from nonpolluting, renewable sources, EVs have the potential to produce no well-to-wheel GHG emissions. On the other hand, EVs powered by electricity generated using coal have the potential to produce more well-to-wheel GHG emissions than gasoline vehicles, depending on the power source. To determine your region’s specific fuel mix, as well as the emissions rates of electricity based on your zip code, see the U.S Environmental Protection Agency’s Power Profiler (http://www.epa.gov/cleanenergy/energy-and-you/how-clean.html). While factors such as the time of day and season can affect which specific power plant (and fuel mix) provides the vehicle’s electricity, these average emissions rates help approximate the impact of these vehicles.
PHEVs typically operate either in all-electric mode or using an internal combustion engine (ICE) and electric drivetrain in a manner similar to a hybrid electric vehicle (HEV). The emissions vary based on the percent of time that the vehicle is in each operating mode. When operating in all-electric mode, emissions are considered from the source that produced the electricity. When the vehicle’s ICE is running, both the upstream and tailpipe emissions must be taken into account. The tailpipe emissions will vary depending on vehicle efficiency.
Comparing EV and PHEV Emissions to ICE Vehicle Emissions
The following table estimates the well-to-wheel GHG emissions associated with a 100-mile trip in four comparable compact sedans, based on the national average for electricity production emissions.
Vehicle GHG Emissions (pounds of CO2 equivalent)
EV 54 lb CO2e
PHEV 62 lb CO2e
HEV 57 lb CO2e
Conventional Gasoline 87 lb CO2e
The Alternative Fuels & Advanced Vehicle Data Center (AFDC) Compare Electricity Sources and Annual Vehicle Emissions tool (http://www.afdc.energy.gov/afdc/vehicles/electric_emissions.php) allows users to determine an estimate for annual well-to-wheel GHG emissions for an EV, PHEV, HEV, and conventional gasoline vehicle based on the electricity production fuel mix in their area.
In addition, Fueleconomy.gov and EPA’s Green Vehicle Guide (http://www.epa.gov/greenvehicles/Index.do) provide annual emissions estimates for individual vehicle models.