SEPTA's board approved on Thursday the construction of rooftop solar systems on several sprawling maintenance centers in Philadelphia, a 3.1-megawatt project the agency says would be the city's second largest after the Eagles' solar installation at Lincoln Financial Field.

SunVest Solar Inc. will own and install the solar systems. SEPTA will pay no upfront costs but will agree to buy the electricity produced for 20 years under a power-purchase agreement.

Under that agreement, SunVest and its partners will finance, design, install, operate, and maintain the roof-mounted photovoltaic systems. The project team includes Solar States, a Philadelphia-based solar-installation company, and New Energy Equity LLC, an Annapolis, Md., renewable-energy financing firm.

SEPTA said the agreement allows it to fix a portion of its energy cost at a predictable level that initially is below what it pays Peco. The renewable energy also will help the agency meet its sustainability goals.

The systems will be installed on SEPTA bus-maintenance centers at 200 W. Wyoming Ave. and the Callowhill facility at 5801 Vine St., as well as the Regional Rail shop at 342 Roberts Ave. and the Fern Rock subway facility at 1100 W. Nedro Ave.

The four buildings combined have about 637,000 square feet of flat roof, or about 14.6 acres. SEPTA says its project is slightly smaller than the Eagles' solar array at the Linc, which is rated at 3.2 megawatts. SEPTA says its project will increase the city's total installed solar capacity by about 30 percent.

"This solar project, along with other exciting new initiatives such as a new fleet of battery-electric buses to arrive this fall, demonstrates SEPTA's enduring commitment to environmental improvement," said Jeffrey D. Knueppel, the agency's general manager.

SEPTA's project will produce about four million kilowatt hours of electricity a year, about 25 percent of the energy needs of the four maintenance centers. Though it is large for a solar project, in regional energy context it will produce as much electricity as the Limerick Generating Station's nuclear reactors in Montgomery County can put out in less than two hours.

Erik Johanson, SEPTA's director of innovation, said that the transit agency had considered installing solar systems when it began its sustainability program in 2010, but that they were too costly at that time. The price of solar panels has dropped significantly since then, he said, and SEPTA recently put new roofs on the buildings, making a long-term commitment to solar power feasible.

SEPTA announced it was seeking bids for the solar systems in December, a month after its board approved the installation of a controversial 8.8-megawatt natural-gas cogeneration plant in Nicetown that had attracted opposition from climate activists, who objected to the use of fossil fuels for power generation. They had urged SEPTA to use renewable power, instead.

SEPTA officials said the decision to move ahead on solar was not intended as a response to the critics.

"I can understand how the timing might look like that, but this is a totally independent project from the [gas plant]," said Johanson.

SEPTA will pay the solar developers 7.2 cents per kilowatt hour for power, and the contract provides for annual price escalations of 1.9 percent, Johanson said. The annual price escalations are standard under most power-purchase agreements.

Four companies bid on its solar proposal, which initially included two other trolley-maintenance facilities later excluded from bidding because they might undergo major renovations in the next 20 years.

According to SEPTA's presentation to its board, the agency twice asked bidders to submit "best and final offers" after they had made their initial offers. Only SunVest submitted bids that SEPTA said gave the agency a positive net present value. The other bidders were SolarCity, WGL Energy, and Tangent Energy Solutions, a Kennett Square company.

"We're very conservative with our assumptions," said Johanson. "We were trying to come up with a project that we felt pretty confident would equal or beat our Peco price over the period of the contract."

SunVest Solar says it has installed more than 13 MW of solar systems nationwide with more than 600 customers, including a 237-kilowatt array it installed last year on the roof of Millbridge Elementary School in Delran, Burlington County.

"I can tell you that the project is extremely competitive and a fantastic deal for SEPTA," Kirk Kindred, national sales director for SunVest, said in an email. "We all want projects in Pennsylvania to work out and, this will help shape the market (we hope)."