A new study from Northwestern University finds that the U.S. transmission grid must be upgraded to ensure that electric vehicles (EVs) are powered by renewable energy. The research, published in Nature Communications, indicates that even if all gas-powered vehicles were replaced with EVs and enough clean energy was generated, existing grid congestion would prevent much of this electricity from reaching charging stations.
“Even if the U.S. fully adopts EVs and generates enough renewable electricity to charge them, it still won’t be enough,” said Adilson Motter, who led the study at Northwestern. “We found the limiting factor for cars to be powered by clean energy has less to do with the availability of renewable energy and more to do with the ability to transmit that energy from generation sites to where it’s needed. The power lines are congested, and that leads to congestion-induced CO2 emissions.”
Motter is a professor of physics at Northwestern’s Weinberg College of Arts and Sciences and directs the Center for Network Dynamics. He worked on this research with Chao Duan, a former Research Assistant Professor at Northwestern.
The researchers compared data on vehicle usage and grid infrastructure using computer models simulating different scenarios of electrification and renewable generation across the country. In every scenario with high levels of EV adoption, they found grid congestion was a significant barrier.
As demand for electricity rises in urban areas due to more EVs, but most wind and solar farms remain far from cities, limited transmission capacity means clean power cannot always reach where it is needed. This forces greater reliance on nearby fossil fuel plants for charging EVs instead of renewables.
In their most ambitious simulation—converting all U.S. vehicles to electric—the team calculated that nearly one-third of potential CO2 emission reductions would not be realized unless grid constraints were addressed.
“The charging schedule of EVs can be optimized to align with intermittent renewable generation,” Motter said. “But even with smart charging, efficient use of clean energy still depends on having enough transmission capacity to deliver it where it’s needed.”
The study recommends increasing transmission capacity by 3% to 13%, which could involve building or expanding high-voltage lines in key locations rather than overhauling the entire system. Targeted upgrades could help deliver more wind and solar power from remote sites into cities where demand is highest.
Motter emphasized focused improvements: “Power grids began as local networks, where consumption was close to generation,” he said. “Over time, they evolved into nationwide — even continent-wide — systems. It was a gradual growth process built on existing infrastructure. No one wants to redesign it from scratch, but we do need targeted upgrades that reflect the large-scale reach of today’s grid.”
The U.S. power system is divided into three mostly independent regions—Eastern, Western, and Texas—with limited ability for interregional transfers; better connections among these could also help distribute clean energy more effectively.
This research received support from Leslie and Mac McQuown through Northwestern's Center for Engineering Sustainability and Resilience, a Resnick Award from the Paula M. Trienens Institute for Sustainability and Energy at Northwestern University, as well as funding from China’s National Natural Science Foundation.