US marginal electricity grid mixes and EV greenhouse gas emissions

Battery electric vehicles (BEVs) are often portrayed as “green,” implying negligible greenhouse gas (GHG) emissions. While BEVs are zero emission vehicles, the electrical power generators used to recharge vehicle batteries do emit copious GHGs. Some analysts have estimated the power plant GHG emissions due to charging EV batteries using the average electrical generator grid mix for a given region. However, the GHG protocol specifies that analysts should use the marginal grid mixes to accurately calculate GHG emissions from adding EVs to the vehicle fleet. This paper utilizes the marginal grid mixes for each electrical power region in the US, and calculates the vehicle-weighted average GHG emissions for the entire country. These calculations demonstrate that, on the average, each BEV that displaces a gasoline hybrid electric vehicle (HEV) will increase GHGs by more than 7% and each PHEV put in service will increase GHGs by an average of 10% compared to a gasoline HEV.

► The GHG protocol recommends marginal over average grid mix to estimate EV GHGs.
► Using average rather than marginal grid mixes inflate the GHG benefits of EVs.
► US battery EVs will increase GHGs by an average of more than 7% relative to hybrid vehicles.
► US plug-in hybrids increase GHGs by an average of 10% relative to hybrid vehicles.
► US fuel cell vehicles decrease GHGs by 25% relative to hybrid vehicles.