SEPTA is aiming to convert 10 percent to 20 percent of its energy consumption to renewable sources, such as solar panels or windmills.

The transit agency, one of the largest energy consumers in the region, uses 480,000 megawatt hours a year of electricity for services such as trolleys, trains, and facilities. All of that power is drawn from the electrical grid, which uses primarily coal, nuclear, and natural gas sources, said Erik Johanson, SEPTA's director of business innovation. About 4 percent now is generated by renewable sources.

On Wednesday, SEPTA issued a request for proposals that could allow it to convert to renewable sources without spending more money on energy, he said. The agency wants to begin purchasing more renewable energy by November 2019, the request for proposal states, and requires a commitment to a fixed price for 10 to 20 years.

SEPTA is leaving open-ended how that could be done. It will consider proposals for new sources of hydroelectric power, for solar or wind power, or for simply plugging SEPTA into existing renewable sources in a 13-state region that stretches as far west as Illinois and Kentucky and from Virginia to New Jersey. Something new, though, may be the most cost-effective route.

"Within the last several years, the price for renewable energy has been dropping," Johanson said. "A new project would be more likely to provide cost-neutral electricity."

SEPTA may also need to blend several energy sources to fulfill its requirements, he said.

A sustainability program adopted in 2011 states that SEPTA has a goal to reduce greenhouse gas emissions from its operations.

SEPTA has been gradually shifting its 1,400-bus fleet from petroleum burners to hybrid vehicles. About half the fleet is now made up of diesel-electric hybrids, and the transit agency aims to increase that to 95 percent by 2021.

The agency has also drawn criticism from environmental groups for investing in an 8,800-kilowatt natural gas power generator in Nicetown. Although natural gas is one of the cleanest forms of fossil fuel, activists have argued that it could add to air pollution in a community already exposed to vehicle exhaust from SEPTA's Midvale Bus Depot, near the Roosevelt Expressway and Hunting Park Avenue. They also note that greenhouse gases can be created by extracting and transporting natural gas. The group is appealing the facility's air permit with the Licenses and Inspections Review Board.

Peter Winslow, a member of 350 Philadelphia, an environmental group that has led the fight against the Nicetown generator, lauded SEPTA on Wednesday for its investigation of renewable energy sources.

"Commitment by SEPTA to obtain 10 percent to 20 percent of its electricity from renewable sources is significant," he said.

SEPTA's effort to begin conversion by next year recognizes the importance of climate change, Winslow said, adding that the agency could go further by retiring renewable energy certificates and insisting that the vendor it hires grow the amount of overall renewable energy being generated.

While SEPTA is seeking to become increasingly green, it is taking an incremental approach out of concern about the cost of certain sources of energy in the future. By not committing to a single energy source, SEPTA can avoid sudden price spikes, Johanson said.

This story has been updated to correctly report Winslow's proposals.