It's an environmental clash that neither side wanted: Solar advocates are squaring off against conservationists.
On one side, fans of solar energy are pushing a cleaner alternative to fossil fuels, with massive solar projects popping up across the United States. On the other, conservationists and people who live near the solar projects are watching in horror as green fields are filled with rows of silicon solar panels, damaging ecologically sensitive areas.
"It's kind of funny to me that there's environmental resistance to wind and solar, which is an environmental solution," said Michael Webber, a professor of energy resources at the University of Texas at Austin.
But, he added, it's not entirely unexpected as solar has gone from an emerging technology to one that's now more mainstream.
"Any time you do anything at scale, you start to get resistance," Webber said. "There's resistance to oil and gas, and nuclear and shopping centers. It's a sign of maturity in solar that, when people want to scale, you get resistance."
The battles have played out state by state and county by county, forcing communities to consider just how much they are willing to sacrifice to decarbonize the economy.
They have also triggered a hunt for new locations to put millions of more solar panels, sometimes in unexpected places and with the help of unlikely allies. Researchers, environmentalists and energy companies are increasingly turning to spots such as agricultural canals, grazing pastures, the roofs and the parking lots of big-box retail stores, the land next to interstate highways and airports, and the tops of landfills, mines and wastewater treatment plants.
If big open spaces aren't right for utility-scale solar farms, the thinking goes, then the panels will have to squeeze in everywhere else.
"We need to look at every previously developed piece of land: every rooftop, every parking lot and, here in California, 4,000 miles of open canal," Jordan Harris, CEO of Solar AquaGrid, said. The startup is in the process of installing solar-panel canopies over water supply canals in California's Central Valley — a project that will have the added benefit of reducing evaporation of scarce water. It has $20 million in state funding.
The price of solar panel equipment has plummeted in recent decades and made solar competitive with fossil fuels. Solar panels are now a common sight on houses, businesses and some government infrastructure.
But analysts still expect most solar energy production in the near future to come from utility-scale projects, in part because of the savings that comes with massive installations.
It's those projects that are facing pushback. Local governments in states such as California, Indiana, Maine, New York and Virginia have imposed moratoriums on large-scale solar farms, as a national push for cleaner energy has collided with complaints about how the projects affect wildlife and scenic views. In one Nevada town west of Las Vegas, residents are trying to block a proposed 2,300-acre solar field.
NBC News counted 57 cities, towns and counties across the country where residents have proposed solar moratoriums since the start of 2021, according to local news reports, and not every proposed ban gets local news coverage. At least 40 of those approved the measures. Other localities did so in earlier years.
That resistance is a threat to the big ambitions of the solar energy movement. An analysis of the U.S. solar market published in December by the research firm Wood Mackenzie and by the Solar Energy Industries Association, a trade group, named "siting restrictions" as one of the constraints on growth, along with supply chain limits and other factors.
At the same time, there's rising urgency to cut down on fossil fuels. The United Nations' Intergovernmental Panel on Climate Change warned last month that global warming was already endangering food and water resources, while the Russia-Ukraine conflict has led to more calls for U.S. energy independence.
It has all left solar advocates to look for nontraditional allies.
Last year, Environment America, an advocacy group, launched a campaign to ask Walmart, the largest U.S. retailer, to commit to putting solar energy systems on nearly all its roofs and parking lots by 2035. By November, more than 150 other environmental organizations had signed on to the push. A study found that putting solar canopies over the parking lots just at Walmart "supercenters" would create enough electricity for 346,000 charging stations for electric vehicles.
"There's only so much land on this planet, and right now those rooftops are doing nothing other than keeping the sun out and keeping the rain out," said Johanna Neumann, senior director for the renewable energy campaign at Environment America, an advocacy group.
"By giving them that dual function of becoming solar plants, we can give them real value."
And, she said, the roofs of big-box stores and warehouses let the solar sector sidestep battles over their environmental impact.
"You don't run into the problem of pitting clean-energy advocates against conservationists," Neumann said.
Solar has already been gaining momentum among corporations seeking to reduce their emissions. Walmart ranks No. 3 in solar capacity among U.S. businesses, after Apple and Amazon and ahead of Target, according to the Solar Energy Industries Association. And as of 2019, there was 15 times more solar capacity installed by U.S. businesses including warehouses and big-box stores than there was a decade earlier, the group said in a report.
Walmart hasn't agreed to the full request of environmental groups, but it told NBC News it has more than 550 renewable energy projects, including solar and wind, implemented or under development. Several have opened recently in California, including with parking lot canopies. The company has a goal of using 100 percent renewable energy by 2035, up from 36 percent by its estimate now.
"Our choice of solar vs. wind and onsite vs. offsite for any given project depends on a broad range of factors including cost and resource availability," Walmart said in a statement.
Smaller solar sites don't have the same economies of scale as utility-size sites do, but experts said they have other benefits, including revenue from selling power, local self-sufficiency, reduced transmission costs and the opportunity for local storage with increasingly inexpensive battery technology.
"The obvious best place to put solar energy is to integrate it directly into whatever you might be powering," said Joshua Pearce, an engineering professor at Western University in Ontario, Canada, who has researched putting solar panels on reservoirs, parking lots and elsewhere.
"Right now, the grid is all centralized, and I think it's going to move toward a federation of different solar communities," he said.
The Biden administration has said that solar energy could account for nearly half of America's electricity by 2050. Solar and wind energy are broadly popular in surveys.
A whole corner of the solar industry has developed to help identify overlooked locations to install panels. Aurora Solar, a tech startup based in San Francisco, sells software to installation companies allowing them to find potential clients and to design systems.
The software pulls data on weather and available sunlight at a given location and combines it with data from aerial imagery and lidar laser technology about the size and dimensions of individual buildings — all to encourage solar adoption on a much wider scale.
It's work that used to be done in person, one building at a time.
"I've literally met people who fell off roofs doing that," said Christopher Hopper, who co-founded the company with a business school classmate, Sam Adeyemo. The old way, he said, "is not scalable. It takes a lot of man-hours to design, and is also not very accurate."
Google has run a consumer-facing website, Project Sunroof, since 2015 to tell people how much solar energy they could create at a given U.S. address.
There's also a push to put solar farms in places that are off the beaten path, or at least away from scenic vistas.
Houston has chosen the 240-acre site of a former landfill to install what the city said will be the largest infill solar project in the nation. In a neighborhood named Sunnyside, the project will generate enough electricity for 5,000 homes, according to the city. Similar projects have been built on landfills throughout New Jersey.
An energy firm is building a solar project on a former coal mine on the border of Kentucky and West Virginia, while in New York state, researchers at Cornell University are testing putting solar panels in a field where sheep graze.
A city in Northern California says it has the largest floating solar farm in the U.S. at its wastewater treatment plant, and in January, a China-based energy company said it had built the world's largest floating solar array on a reservoir there.
And last year, the Biden administration encouraged the development of solar projects on highway right-of-way, with a notice from the Federal Highway Administration telling field offices to work with states on ideas. Researchers at the University of Texas at Austin, including Webber, have said most states have more than 200 miles of interstate frontage suitable for solar development, especially near exits and rest stops.
Creative locations have a particular benefit: fewer potential neighbors who might complain.
"To solve problems, we have to do stuff, and sometimes people don't want to do stuff. It's annoying," Webber said. "We have to decide as a nation whether we're going to invest in the future or not."