This study analyzes the impact of New Energy Vehicle (NEV) promotion on electricity sector carbon emissions using panel data from 50 Chinese cities (2006-2020) and a multi-period DID model. NEV promotion, while reducing transportation sector emissions, has increased carbon emissions in the electricity sector due to reliance on coal-based power generation. Results indicate that (1) In “Ten Cities, Thousand Vehicles” pilot cities, NEV adoption increased power sector carbon emissions by an average of 1.19%. Furthermore, the “Ten Cities, Thousand Vehicles” plan generated significant spatial spillover effects, elevating power sector emissions in neighboring regions. (2) Through China’s “West-to-East Power Transmission” project reveals that the use of electricity by NEVs also places carbon reduction pressure on the power generation sector. This pressure can shift through local and cross-regional electricity transmission, thereby creating a “regional transfer” effect of carbon emissions; (3) Mechanism analysis indicates that NEV usage increases regional electricity demand and fossil energy consumption while synergistically enhancing regional renewable energy technological innovation. Policy implementation has also accelerated advancements in “three-electric systems” (battery, motor, electronic control) and V2G technologies, which improve NEV energy efficiency though require further refinement; (4) Under policy synergies between NEV demonstration cities and low-carbon city initiatives, NEV promotion effectively mitigates carbon emission transfers to power sectors while amplifying environmentally positive externalities. This study provides a comprehensive assessment of the negative environmental externalities of NEV promotion, offering a new perspective on NEV development.