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Summary: Under some circumstances, electric vehicles (EVs) can reduce overall environmental impacts by displacing internal combustion engine vehicles (ICEVs) and by enabling more intermittent renewable energy sources (RES) by charging with surplus power in periods of low demand. However, the net effects on greenhouse gas (GHG) emissions of adding EVs into a national or regional electricity system are complex and, for a system with significant RES, are affected by the presence of storage capacity, such as pumped hydro storage (PHS). This article takes the Portuguese electricity system as a specific example, characterized by relatively high capacities of wind generation and PHS. The interactions between EVs and PHS are explored, using life cycle assessment to compare changes in GHG emissions for different scenarios with a fleet replacement model to describe the introduction of EVs. Where there is sufficient storage capacity to ensure that RES capacity is exploited without curtailment, as in Portugal, any additional demand, such as introduction of EVs, must be met by the next marginal technology. Whether this represents an average increase or decrease in GHG emissions depends on the carbon intensity of the marginal generating technology and on the fuel efficiency of the ICEVs displaced by the EVs, so that detailed analysis is needed for any specific energy system, allowing for future technological improvements. A simple way to represent these trade‐offs is proposed as a basis for supporting strategic policies on introduction of EVs. [ABSTRACT FROM AUTHOR]