It’s not just using a handheld phone while driving that’s a menace to society. It turns out that charging it in the car has consequences, too.

That’s because a phone drawing electricity from a USB port cuts 0.03 miles from each gallon of gasoline in a tank. Across the fleet of vehicles in the United States, that would mean about 970,000 tons of extra planet-warming carbon dioxide a year, according to calculations by Jon Bereisa, a retired General Motors engineering executive who studies vehicle power usage. With a race underway to see how many charging ports automakers can cram into a car, the increased pollution is only going to get worse.

“Do I think we’re at peak USB? No,” said Mary Gustanski, vice president of engineering and program management at Delphi Automotive, which makes wiring and USB ports for vehicles. “We’ll get more and more creative to not only allow you to connect with USB but also to connect wireless. Consumers want their car to be just like their home.”

It’s not just an environmental issue, either. The proliferation of consumer devices, the growth of dashboard touch screens and other technology, and the shrinking size of engines to meet fuel-economy mandates mean the 12-volt automobile electrical system is just about tapped out. Some carmakers are already turning to supplemental 48-volt systems in future models.

The number of vehicles sold in the U.S. with USB charge ports rose to about 14.6 million last year from about 3.3 million in 2005, the first year they were available, and is projected to climb to 16.7 million by 2022, according to a forecast from the consulting firm IHS. Global sales of vehicles with USB ports will rise to 85 million in 2022 from about 49 million last year, IHS said.

To make his calculation, Bereisa assumed that a typical smartphone connected to Wi-Fi or the Internet needs about 4.8 watts of energy to charge in a car. (Delphi estimates that some less-efficient models draw twice that amount.) For a vehicle getting about 30 miles per gallon, that’s a 0.03 mile-per-gallon loss, he said. Spread out across about 3 trillion road miles that motorists drive each year in the U.S. — assuming an average speed of 30 mph — the estimated extra usage is 100 million gallons of gasoline, or about $200 million in costs, said Bereisa, the CEO of Auto Electrification LLC in Sunrise Beach, Mo.

The estimated extra CO2 created by plugging in one phone in every car in the U.S. would be about the same as that produced by 185,257 passenger vehicles in one year, according to an Environmental Protection Agency website that converts greenhouse gas into real-world equivalents. Put another way, that’s the pollution created by burning 945 million pounds of coal.

By far the cheapest way to charge a smartphone is at home, Bereisa said. With gasoline at $2 a gallon, it costs about 2 cents an hour to charge a phone in a car compared with about 0.06 cent at home, or 33 times less.