The Washington Post is very skeptical of President Donald Trump’s proposal to cover the U.S.-Mexico border wall he’s promised to build with solar panels.

The paper was quick to point to past reporting on the pitfalls of using solar panels to help pay for a border wall. It’s a big turn from WaPo’s usually positive stance on solar panels as a way to fight global warming.

“And I’ll give you an idea that nobody has heard about yet,” Trump said at a rally in Iowa Wednesday night.

“We’re thinking about building the wall as a solar wall, so it creates energy, and pays for itself,” Trump said. “And this way Mexico will have to pay much less money, and that’s good.”

“Pretty good imagination, right? Good? My idea,” Trump said.

WATCH:

President Trump proposes a “solar wall” at the US-Mexico border that could “pay for itself” https://t.co/fLSi9N8Mdo pic.twitter.com/mbqfbCaP34 — CNN (@CNN) June 22, 2017

WaPo rightfully points out the idea wasn’t likely Trump’s own. Las Vegas businessman Thomas Gleason submitted a bid to the Department of Homeland Security in April to top the 2,000-mile border wall with solar panels.

Gleason said this could be done cheaply and would pay for itself in under 20 years. Axios reported earlier this month that Trump pitched the solar panel idea to congressional leaders in a closed-door meeting, though it’s not clear how serious he was.

Now, with media reports abounding on his solar wall suggestion, Trump has publicly backed the idea. WaPo used the opportunity to highlight the pitfalls of putting solar panels atop of border wall.

WaPo’s Dino Grandoni wrote that “experts who have taken the solar-paneled border wall proposal seriously say such a structure would have significant issues.”

“Vertically fixed panels could lead to an efficiency loss of around 50 percent,” Grandoni wrote, referring to a Financial Times article from February.

Grandoni also pointed to a past WaPo report by Sophie Yeo, which noted that “solar panels degrade over time” and the “requirements dictated by the security aspects of the border wall — bricks and spray paint, for example — could further reduce efficiency.”

“With less than 2 percent of the U.S. population living within 40 miles of the Mexico border, the electricity generated by the wall would mostly be useless — unless costly transmission lines were built to take the electricity to other areas of the country,” Yeo reported.

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