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The four vehicles parked at a depot in South Burlington, Vt., look no different from the yellow school buses familiar to millions of schoolchildren. But beneath their steel shells, these buses are packed with technology that could be vital in the transition to clean energy.

While their main job remains transporting children, the vehicles take on a second task while sitting idle during school hours. The local utility puts their batteries to work, storing excess renewable energy so it can be pumped back into the grid when needed.

The buses are a test of the idea that electric vehicles, which skeptics often see as an expensive burden that could bring down electric grids, could be just the opposite: a buffer that soaks up power when there is too much and provides it when demand for electricity surges.

Any suitably equipped electric vehicle can be used to store surplus electricity, avoiding the need for utilities to fire up gas-fueled power plants when there isn’t enough sun or wind. But school buses work especially well because they have big batteries and spend much of the day parked.

“There’s no better tool than an electric school bus fleet to sort of smooth those curves,” said Duncan McIntyre, the chief executive of Highland Fleets, a company near Boston that provides the buses and equipment. Synop, a New York firm, provides the software to manage the interaction between vehicles, chargers and the grid.

Utilities across the country have been testing the ability of batteries in electric vehicles to help stabilize increasingly unreliable power plants and lines that have faltered under stress from hurricanes, heat waves and other extreme weather linked to climate change.

Grueling summer heat this year tested the Texas grid for weeks, forcing officials to plead with homeowners and businesses to use less energy so the state could avoid rolling blackouts or the kind of power failure that left millions of people without light or heat in 2021.

Some energy experts say one solution to these problems is to corral thousands of rooftop solar panels, home batteries and electric vehicles around a city or state into virtual power plants. Tethered together with the help of software, the collective capacity of such devices to generate and store energy can be more than enough to avoid a blackout when power plants falter or strong winds take down a transmission line.

Grids primarily use power plants with quick start abilities, known as peaking units, to serve as backup sources of energy. But such plants typically use gas, a major source of greenhouse gas emissions, and are expensive to run and maintain. Many have also failed to perform when they are most needed.

Electric school buses, in particular, could be very useful to the grid because of their limited use during school days and broad availability during summer’s sweltering temperatures.

Schools in every state except Wyoming have committed to begin using electric buses, though the number on the roads is small at fewer than 3,000 as of June 30, according the World Resources Institute, a nonprofit that works on the energy, environment and related issues.

California’s total number of school buses leads the nation, and Montgomery County, Md., has more than any other school district.

“We’re on the edge of the technology, which doesn’t happen often for school buses,” said Daoud Chaaya, vice president of sales for Thomas Built, a unit of the truck maker Daimler that supplied South Burlington’s electric buses.

The World Resources Institute is pushing U.S. policymakers to make sure all school buses are battery-powered by 2030, a goal that would also reduce asthma and other diseases in children by eliminating pollution from combustion-engine buses.

“There’s definitely a lot of challenges,” said Sue Gander, director of the institute’s electric school bus initiative. “It’s going to take some time for everybody to get there.”

Cost remains a big hurdle: An electric school bus can cost three times as much as a $100,000 diesel bus. The Bipartisan Infrastructure Law, passed in 2021, allocated $5 billion over five years to help schools buy electric buses, and the price is expected to fall in coming years. In the meantime, school districts can defray their expenses by letting utilities use buses to store energy.

In South Burlington, the school district leases the electric buses from Highland, which also supplies equipment to recharge them and pays the electricity bills. Those bills are lower than normal because of a deal that lets Green Mountain Power, the utility serving most of Vermont, draw power from the bus batteries when demand surges. They are part of a network that also includes batteries that homeowners install to provide backup power during blackouts.

In total, Green Mountain Power has access to 50 megawatts of battery storage from school buses, home batteries and other sources, said Mari McClure, the utility’s chief executive. That’s as much as a small gas power plant. Unlike a plant that runs on fossil fuels, the power is available almost instantly.

The utility asked Vermont regulators last month to allow it to install batteries at the homes of its customers who don’t already have one, an effort that would dovetail with its work on school buses.

Over time, Ms. McClure said, enough electric school buses and home batteries may be connected to the grid to stop her utility from needing to buy electricity from out-of-state power plants. Commercial vans, pickups and garbage trucks could join the network as more businesses and cities buy electric vehicles.

But linking these vehicle batteries to the grid will take not only time but also money. While a standard electric vehicle charger for buses can cost $3,000 to $7,000 to install, initial data from early demonstrations of electric pickups indicates that equipment needed to send power back to the grid ranges from $10,000 to $58,000, according to the Electric Power Research Institute, an independent nonprofit organization. Utilities may also need to upgrade their power lines, transformers and other equipment.

There are also difficult legal and financial issues to sort out. Many states have struggled to determine how to compensate homeowners and businesses for power they deliver to the electric grid from batteries and rooftop solar panels.

Energy experts said that such issues would get addressed and that the high costs of electric vehicles would come down as utilities, regulators and manufacturers gained more experience.

Vehicle batteries can meet some of the needs of customers, utilities and the wholesale market for electricity, said Daniel Bowermaster, senior program manager for electric transportation at the Electric Power Research Institute. “From the technological standpoint, those things are within the realm of possibility.”

Officials in South Burlington, whose diesel bus fleet is mostly paid off, said they were willing to spend more on electric buses The new buses are much better for the environment and public health, said Tim Jarvis, the school district’s finance manager.

There are other benefits, too.

Sean McKenzie, transportation coordinator for South Burlington schools, who drives a bus because of a labor shortage, said children no longer had to shout above the roar of a diesel engine.

“I was surprised that they were quieter,” he said.