SEPTA’s 22 electric, emissions-free buses are on the street, but it remains to be seen whether they’re a first step toward greener public transit or a novelty that will prove unsustainable.

The authority’s $23.8 million investment in 25 electric buses — three more will be put in service soon — from the manufacturer Proterra represents the biggest commitment yet to the technology by an East Coast transit agency. They stand out from the rest of SEPTA’s buses, with big front and rear windows, and a Peco advertisement wrapped around them.

The new buses, though, compose a small percentage of the transit agency’s almost 1,500-bus fleet, and SEPTA won’t commit to going fully electric. Electric buses used elsewhere have proved less reliable than their emissions-spewing predecessors, and it is unclear whether the buses’ cost and SEPTA’s battery charging capacity will prevent them from being used widely in a big city.

“We don’t know how they perform in our conditions,” said Erik Johanson, SEPTA’s director of innovation. “We need to go through all four seasons to see how these buses are able to perform.”

Public transportation is a double-edged sword for the environment. A bus filled with 77 people, the capacity of SEPTA’s new buses, potentially represents 77 fewer cars on the road and a reduction in the polluting emissions they produce.

>>READ MORE: SEPTA will use in-house talent to remake Philadelphia’s bus network

But those buses are polluters themselves.

Nationwide, the transportation sector produces 27 percent of the country’s greenhouse gas emissions, according to a 2018 study from PennEnvironment. Airplanes, passenger cars, and boats are included in the broad category of transportation, and buses are a small percentage, but public transit buses rely heavily on diesel fuel. About 64 percent of SEPTA’s fleet are hybrid vehicles, which use less diesel, with 300 more to be delivered by 2021. But those are only slightly more fuel efficient than a bus powered solely by diesel. Bus emissions also contribute to air pollution, including ozone, or smog. The American Lung Association gave Philadelphia an F rating in its measure of ozone in the city released in April. The city’s air quality can be particularly harmful to children, asthmatics, or people who exercise outside.

Electric buses promise all the environmental benefits of public transportation with minimal pollution of their own. SEPTA estimates its new electric buses would produce 2.3 pounds of CO2 per mile, not through emissions but as a byproduct of the electricity needed to charge the vehicles’ lithium-ion batteries. A hybrid bus produces 6.2 pounds of CO2, and a diesel bus produces 7.2 pounds on equivalent routes, SEPTA officials said. The average passenger car, by comparison, produces about 0.89 pounds of CO2 per mile, according to the Environmental Protection Agency.

“Obviously the electric buses are a lot cleaner in terms of their quality impacts,” said Joe Minott, executive director and chief counsel for the Clean Air Council, a Philadelphia-based environmental group.

Buses are SEPTA’s only fossil fuel-powered vehicles. Regional Rail, subways, and trolleys all run on electrical systems. SEPTA also has 38 trackless trolleys, which run on overhead wires like a trolley.

SEPTA has struggled to meet its sustainability goals. In 2017, its goal was 0.5 pounds of CO2 emissions per passenger mile traveled across all its modes of travel. It barely missed that goal, reporting 0.56 pounds per passenger mile traveled. That’s largely due not to an increase of emissions from its vehicles, but because of declining ridership, particularly on buses. The environmental benefits of public transit shrink as fewer people use it.

“If people are not riding our system, we’re not able to meet our goals,” said Becky Collins, SEPTA’s corporate initiatives manager for sustainability. "Buses need more people on them to be more efficient. "

>>READ MORE: SEPTA keeps bleeding bus riders. It may take years to stanch the losses.

Electric buses, though, can’t match the reliability of fuel-powered vehicles. The buses SEPTA bought are touted as getting about 350 miles on a single charge. It is highly unlikely the buses will consistently, or even frequently, perform so well, Johanson said. Their batteries are vulnerable to cold. A January CityLab story reported that on 2018′s Super Bowl Sunday, a 5 degree Fahrenheit day in the city, an electric bus managed to hold a charge for 40 minutes while traveling 16 miles. Hot weather can also be a problem, since air-conditioning adds to the drain on the power source. The load the bus is carrying, and terrain with steep inclines, can also affect the batteries’ performance, according to a May report from the World Resources Institute, an international research organization focused on natural resources.

The World Resources Institute profiled SEPTA specifically, and found the agency’s electrical capacity would be able to sustain only 105 more electric buses. SEPTA had to install 25 charging stations at the Southern District Bus Depot, the home for all its electric buses, and a new substation to handle the power needs. The agency would need to invest in new transformers, substations, and sources of electricity at its eight bus depots before electric buses could become a viable replacement for its current hybrid and diesel fleet, the report stated.

The buses themselves are also expensive. SEPTA’s green energy plan requires the agency to not spend more than it normally would for resources like new vehicles. Getting the electric buses, which cost $150,000 more than a hybrid bus, was only possible because a $2.6 million grant from the Federal Transit Administration’s Low or No Emission Vehicle Competitive Program covered the difference, SEPTA officials said.

Minott would like to see SEPTA more aggressively commit to electric vehicles, but understood why the agency was embracing the technology slowly. “It makes sense that they are cautious, and I believe they are committed to moving forward,” he said.

SEPTA is running the buses on two routes, the 29 and 79, which both travel between South Philadelphia and Center City. The routes each put about 100 miles on a bus a day. The territory is also friendly for electric buses, Johanson said. “They’re short and they’re flat,” he said of the routes.

Despite the obstacles, SEPTA believes electric buses could become viable. It is buying 10 more buses from New Flyer to be used at its Midvale Bus Depot, though that’s 15 fewer buses than originally planned due to the power issues.

When it began investing in the technology, the batteries available at the time would have likely required intermittent charging during service hours. During the procurement process, Proterra introduced a battery capable of handling longer distances, and SEPTA switched to buses that only need to be charged overnight.

“The technology keeps on improving,” said Collins. “It just keeps on getting better and better and better.”