CHARLOTTE — How many pigs does it take to change a light bulb? We don't know either, but apparently a few together can turn one on.

Duke Energy is reaching into its piggy bank to buy more poo from pigs to make power.

Duke is required by law to use pig waste as a renewable energy source.

They had already been importing gas from the waste from other states, but they're hitting an important milestone in a long road to meet their goals.

“We've contracted with a company that is going to collect the swine waste in eastern North Carolina, extract the methane gas from it,” said Duke spokesperson Randy Wheeless.

From there, Duke Energy plans to pump the gas to four gas-burning power plants across North Carolina.

The program goes back nearly a decade to a law taking aim at North Carolina's growing pile of pig and poultry waste.

Duke has to ramp up how much gas they make under the law. Eventually, it's supposed to make up about 0.2 percent of their entire load.

Wheeless says this step will power about 10,000 homes per year, not nearly the required amount by law.

But the company's had issues getting that much.

“In the past, a lot of the projects had a power plant built where the swine waste was, which makes sense. But it was way too expensive when you ran the numbers,” Wheeless said.

Tim Profeta, director of Duke University’s Nicholas Institute for Environmental Policy Solutions, has been studying the program as the utility developed the technology.

He says this step puts them well on the way to meeting requirements, plus it could be better for the environment than leaving the waste alone.

“Methane is a very powerful greenhouse gas contributing to global warming. It's probably about 25 times more powerful than CO2 (carbon dioxide), so when you burn it, you knock down its global warming potential,” he said.

That's because when you burn methane, you get carbon dioxide and water.

“So in a way, it's carbon neutral electricity, because we have emissions. We know that. But it's no more than what the swine waste would produce just left naturally,” said Wheeless.

And it's a renewable fuel that swine and poultry do indeed just leave naturally.

Duke says the program doesn’t have a site selected yet, but it should be on line in 2017.