When President Trump said last week that the U.S. would continue to be "the cleanest" country on Earth despite pulling out of the Paris climate accord, building a solar-paneled border wall probably wasn't what most people had in mind. But, apparently, that could be part of his plan. 

Axios reported today that Trump pitched Republican congressional leaders on building a border wall covered in solar panels, and to use the electricity generated to cover the costs. 

Trump envisions constructing 40-foot to 50-foot tall barriers, with solar installed all over, so they would be "beautiful structures," according to three people with direct knowledge of the meeting. 

According to Axios:

"The president said that most walls you hear about are 14 feet or 15 feet tall, but this would be nothing like those walls. Trump told the lawmakers they could talk about the solar-paneled wall as long as they said it was his idea. One person cautioned that the president wasn't presenting the solar-paneled wall as the definite solution."

On March 17, the U.S. Department of Homeland Security issued two requests for proposals for border wall design prototypes. One RFP requests submissions for a solid concrete wall. The second, labeled “other border wall,” calls for alternative designs.

One applicant, Gleason Partners LLC of Las Vegas, submitted a proposal to cover the wall with solar panels, the AP reported in April. The panels would power lighting, sensors and patrol stations at the border site. Electricity sales from the wall array, to U.S. utilities and potentially to Mexico, could cover the construction costs in 20 years or less, according to Gleason Partners.

“I like the wall to be able to pay for itself,” said managing partner Thomas Gleason.

The Trump administration could select winning companies in the border wall bid sometime this month.

“If you really believe that putting solar on the border wall would make it 'pay for itself', that means you believe in the positive economics of solar,” said Shayle Kann, vice president of GTM Research. “So why not put solar on all government buildings and new construction?”

He added that the hypothetical "solar wall" is a distraction. "What actually matters is the wall itself, and that's where the conversation should be focused,” said Kann.

What would the solar wall cost?

A few clean energy experts have entertained the idea of a solar border wall in recent months, coming up with their own calculations for what such a wall would cost.

According to Elemental Energy, a locally owned and operated solar PV design and installation firm in Portland, Oregon, Trump’s solar barrier would cost between $10 billion and $15 billion, assuming the project is installed at $1 per watt -- which is the going rate for a growing number of utility-scale fixed-tilt PV systems.

To get to this number, the company assumed that panels would cover 1,000 miles of wall space, since that’s roughly the amount of unobstructed land between the U.S. and Mexico, and that the array would be installed five modules high. Assuming a certain type of solar panel, system efficiency and pricing, Elemental Energy determined that the wall could host a 1.4-gigawatt array and produce 7.28 gigawatt-hours of electricity per day, worth $106 million in electricity sales per year, assuming a $40 per megawatt-hour PPA.*

An earlier version of an Elemental Energy blog post noted the payback could take roughly four years. However, an updated blog post, based on the figures above, would put the payback time for a $10 billion wall, producing $106 million in annual revenue, at roughly 100 years. The timeline would extend to 200 years if the cost of the wall increases to $20 billion, as government reports say it could.

Jigar Shah, co-founder of Generate Capital, calculated that a border wall could host nearly 5,000 megawatts of solar panels that would generate more than 6,600,000,000 kilowatt-hours of electricity. He estimated the project would collect roughly $15.8 billion over the 40-year lifetime of the panels. President Trump has said the wall could be built for around $12 billion.

"So if the power were to be sold to the Mexican people -- power they desperately need -- then the [president] could actually make good on his promise to say that the Mexican people paid for the wall," Shah recently wrote, albeit somewhat tongue-in-cheek.

The numbers may all sound well and good, but there are some logistical issues that make building a solar wall very difficult, and unlikely to come to fruition.

For one thing, these calculations assume standard solar costs. However, a solar wall would likely be more expensive due to electrical issues, transmission costs and possible security measures that would need to be factored in.

The answer to who buys the power isn't straightforward either. Panels would be generating across the entire southern U.S. border, so there would need to be power lines to wherever the demand is. Building and paying for these lines is not easy. Permitting and interconnection would likely be a nightmare for border wall solar developers. 

While all of these issues could be dealt with, in theory -- the U.S. did build a massive, coast-to-coast highway system after all -- the Trump administration would have to do the work to solve all of these challenges, while securing funding from Congress. As sources report, installing solar panels is not a "definite solution" for funding the border wall, and it may never become one. 

This story has been updated to reflect changes in Elemental Energy's calculations. The company no longer states the solar wall could pay for itself in four years.