Chicago has sweltered under several heat waves this summer, pushing the city’s power grid toward its capacity each time.

The manufacturer of the city’s new fleet of electric buses says the vehicles could help relieve some of that burden on the grid.

“Proterra’s Catalyst buses can act as a grid resource, although it is up to individual transit agencies, like CTA, to determine how to utilize those capabilities,” Karaline Bridgeford, a Proterra spokesperson, said.

The company introduced new high-power charging options in May that include energy management capabilities and are compatible with Chicago’s expanding fleet of electric buses.

Bridgeford said the new charging systems were designed for two-way flow of power, which means when the buses aren’t being driven, they could be plugged into the grid and used by ComEd as a battery.

“Every bus on the road will carry about a half a megawatt hour of energy storage onboard, which can be utilized in emergency-response situations, or in periods of high-energy demand to eliminate the need to turn on combustion power-plants,” Bridgeford said. “Proterra also designed its bus batteries so that at the end of their transit life they have useful second-life applications for energy storage.”

Officials with the Chicago Transit Authority say that the buses will charge for five to 10 minutes at end of the route and during layovers, but they are looking at how the fleet could be used to balance out the grid. “We are considering electric grid management options such as off-vehicle energy storage systems,” said Steve Mayberry, CTA spokesperson.

Still, questions remain about the viability of a small fleet of electric buses to serve as a useful grid battery. Susan Mudd, attorney and senior policy advocate for the Environmental Law and Policy Center, says the case for electric transit buses isn’t as clear, as say, the case for electric school buses. Electric school buses have a fixed schedule: drop kids off at school in the morning and pick them up in the afternoon, and they have summers off for the most part.

“Transit buses run more hours a day,” Mudd said. “So, there’s less time when they’re actually not in use and able to feed electricity back into the grid. That may be possible overnight, but there also needs to be recharging.”

Still, Mudd and the ELPC are supportive of CTA’s decision to move toward a cleaner fleet of buses. She says that buses spend a lot of time idling and the emissions are harmful for public health. “They’re harmful to the people standing around, who are waiting to get on and off, and passengers, the driver, and neighborhoods through which they run,” she said.

She added that as the electric grid, over time, gets more and more renewable resources, the emissions reductions will only increase as the source of the buses power will be cleaner energy, too. The 20 new buses are expected to displace 2.5 million gallons of diesel in their lifetime and eliminate 4.5 million pounds of carbon emissions each year.

Chicago already has two electric buses, which have been in service since 2014. The 20 new electric buses are a large investment for the city, but fewer than it once considered. The city put out a call out for bids last year for as many as 45 electric buses.

Chicago Mayor Rahm Emanuel has touted the new buses as greener and more efficient. “This is just the latest example of the types of investments we will continue to make in the years to come, further solidifying Chicago as a world-class city that is at the forefront of modern and green technologies,” Emanuel said in a statement.

The first of the new buses are expected to hit the streets this year, while more questions need to be answered before the city decides what if any role they will play backing up the city’s electric system.