SMUD and seven automakers will demonstrate Thursday a new energy-saving tool that will send a message directly to electric vehicles, postponing their battery charges until after peak hours, unless the owner overrides it.

The tool is driven by the growth in electric vehicle sales. The number of electric vehicles on U.S. roads is growing quickly, with 96,702 purchased in 2013, nearly double the number sold in 2012. Charging each of those automobile requires as much energy as two or more home air conditioners, said SMUD program manager Dave Hatfield.

While the demand from EVs is not very onerous now, he added, history provides a valuable lesson on what can happen when utilities don’t plan for the rapid growth of products that consume a lot of electricity.

“You look back to the ’70s and ’80s when people started putting in air-conditioning instead of swamp coolers here in the Sacramento Valley,” Hatfield said, “and what you saw was a lot of system overload. They had to upgrade transformers, upgrade distribution lines.

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“We’re in the same position now. These cars are coming in fast enough that we need to change the way we do business and the way the vehicles charge.”

Both SMUD and the carmakers emphasize that car owners, not the electric utility, have ultimate control. Customers must opt in to allowing the utility to postpone charges, said Ford Motor Co.’s David McCreadie. Even after opting in, they will receive a message in their car’s in-dash controls or via a mobile app that will alert them about the scheduled delay in charging.

If owners need to power up immediately, he said, they can. The drawback is that, by using power at that time, they’ll be paying the peak rate.

Although this new communications platform is being demonstrated at SMUD, the entire electric utility industry has participated in its development through the nonprofit Electric Power Research Institute. The utilities worked closely with the eight automakers and information-technology developer Sumitomo Electric.

Thursday’s event has both Hatfield and McCreadie excited.

“We will show how all seven different manufacturers’ vehicles, more or less simultaneously, are able to pause or start their charging, and ,” said McCreadie, Ford’s manager for EV infrastructure and smart grid. “They would reinitiate charging after a certain interval. It’s really the first time that’s ever been shown with such a diverse number of vehicles.”

Since each automaker has a different communications system, Hatfield said, SMUD will first show how each car responds individually to the signal and then it will show how all seven respond.

Typically, he said, EV owners will get a message a day in advance to explain that SMUD expects energy loads to be at peak capacity at a certain time and will pause charging. Owners can override the message from SMUD’s smart grid at any time.

The SMUD demonstration will feature a Ford Focus Electric, General Motors Chevy SPARK EV, BMW i3, Mercedes-Benz B-Class ED, Toyota Prius Plug-In, Honda Fit EV and a Mitsubishi i-MiEV. The public is welcome to watch at 9 a.m. at the SMUD customer service center, 6301 S St., in Sacramento.