Plugging in an all-electric car today may blow the fuse box in your home or even black out the neighborhood — and your utility has no way to anticipate or prevent it.

Utilities, car companies, “smart meter” startups and others are struggling to make sure that tomorrow is different.

As carmakers prepare to roll out a new generation of all-electric cars, the San Francisco Bay Area’s electricity system, for instance, is far from prepared. By one assessment, the local electrical grid can handle no more than a few thousand electric cars charging up at peak hours — and that assumes the cars are distributed evenly at about one per block.

“It’s the equivalent of adding another house to a transformer for a short period of time,” said Cree Edwards, CEO of eMeter, which makes software to help utilities manage power. “The draw it has is significant.”

To charge up its battery in just a few hours, an electric car needs to draw a lot of power at a fast rate. The all-electric Tesla Roadster, for example, which can go more than 200 miles per charge, draws an amount of power “that could be as much as an entire house in the summertime at peak” in order to charge in 3½ hours, said Andrew Tang, senior director of PG&E’s smart energy web division. Some Roadster owners have blown home fuse boxes and had to have wiring from their neighborhood transformer replaced, Tang said.

This could be especially troublesome in neighborhoods where multiple electric vehicle owners will live.

Pushing the limits

Utilities have historically determined the size and capacity of electricity transformers, which distribute power to a group of users, based on modelling of expected peak usage. In residential areas, 10 to 16 homes are usually hooked to a single transformer. Transformers in commercial areas can handle larger loads.

Big, flat-screen TVs and more computers have made electricity use per household harder to predict because they are turned on and off and used at different times throughout the day.

That means many transformers are already near their limit at the busiest periods, which can extend through the day until 10 p.m. Two electric vehicles plugged into the same transformer when electricity use in the neighborhood is high could blow the transformer and black out the block, said Edwards and utility and electric car executives.

Utilities will have a better idea of the potential problem next year as the popularity of electric cars increases.

San Carlos, California-based Tesla Motors just produced its 1,000th $100,000-plus Roadster. Nissan, Chevrolet and other major manufacturers will start releasing their more affordable all-electric cars this year. Nissan and other automakers have predicted that electric cars will make up 10 percent of the U.S. market by 2020.

The Electrification Coalition, whose members include Peter Darbee, CEO of PG&E, and Nissan CEO Carlos Ghosn, supports government initiatives that would lead to electric cars making up 75 percent of all vehicles in the United States by 2040.

Plugging plug-in hybrids