Plugging in an electric vehicle is, in some cases, the equivalent of adding three houses to the grid. That has utilities in California—where the largest number of electric vehicles are sold—scrambling to upgrade the grid to avoid power outages.

Last year in the United States, only about 50,000 electric cars were sold. And researchers at the U.S. Department of Energy’s Pacific Northwest National Laboratory have calculated that the grid has enough excess capacity to support over 150 million battery-powered cars, or about 75 percent of the cars, pickups, and SUVs on the road in the United States. But there’s a catch. While power plants and transmission lines have excess capacity, things can get tight when it comes to distributing power to individual neighborhoods. And this is especially the case since electric vehicle sales aren’t evenly distributed. In California, for example, they’re taking off in Silicon Valley and places such as Long Beach and Santa Monica.