By Sophie Yeo, The Washington Post

President Donald Trump has come up with a new idea on how to cover the costs for a proposed border wall between the United States and Mexico: build it with solar panels.

At a White House meeting Tuesday, Trump floated the concept of “beautiful structures,” 40 to 50 feet high, that generate clean electricity from the sun – and would help cover the cost of the project, according to comments reported by Axios.

The U.S. border with Mexico is almost 2,000 miles long. Trump has said his wall, designed to prevent immigrants from crossing into the United States illegally, will cover 1,000 miles, with natural obstacles doing the rest of the work.

That’s a lot of solar panels, potentially generating a significant amount of energy. But the realities of building a 1,000-mile wall covered with solar panels – and then getting that electricity to market on either side of the border – are not so simple.

With few actual details about the design of the wall, the cost of building it or the price that would be paid for the electricity, it is difficult to make any realistic conclusions about the impact of Trump’s solar wall – assuming it ever gets built.

Predictions vary dramatically based on what assumptions about solar wall construction are factored in to the calculations.

Tom Gleason, owner and founder of a company that submitted a proposal to build a solar border wall, has said that his design could generate two megawatts of electricity per hour, would cost about $6 million per mile to build and would pay for itself in 20 years.

Gleason Partners in April released an image of what the wall could look like.

An estimate from Elemental Energy, a solar installation firm based in Portland, Oregon, found that 1,000 miles of solar wall could generate 2,657 gigawatt hours of electricity annually, which would be worth $106 million.

But the wall design itself could prove problematic for a solar-panel installation, according to an analysis in the Financial Times.

Fixing the panels vertically could lead to an efficiency loss of around 50 percent, the analysis says, with the angle at which the sun would hit the wall losing an additional 10 percent in efficiency.

Given that an average solar panel operates at about 20 percent efficiency, and factoring a few other challenges, this leaves the solar border wall operating at just 8 percent efficiency. That’s a big disadvantage.

In addition, solar panels degrade over time. The requirements dictated by the security aspects of the border wall – bricks and spray paint, for example – could further reduce efficiency.

Then there is the question of finding a market for any electricity that would be generated by a solar wall in a remote section of the country.

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.

According to a 2012 study from the Institution of Engineering Technology, a professional association based in Britain, an overhead transmission cable has a lifetime cost of around $8.8 million per mile, with underground cables shooting up to $50.2 million per mile.

This means, to transmit power from the Mexico border to North Dakota, Trump would have to factor in costs of more than $8 billion, at least – in addition to the cost of the solar array, the wall and workmanship.

Alternatively, the power could go the other way, helping to power homes in Mexico – thereby fulfilling another presidential promise, suggested Jigar Shah, founder and former chief executive of SunEdison, a solar-energy company.

“If the power were to be sold to the Mexican people – power they desperately need – then the President-elect could actually make good on his promise to say that the Mexican people paid for the wall,” he wrote in a January blog post.

Shah elaborated on his idea in an interview with The Washington Post.

“In this case, the monopoly utility company of Mexico is CFE,” he said. “CFE is an arm of the government, and if they sign a 20-year contract to buy the power at 6 cents a kilowatt hour, then that technically would be coming from the government.”

But it is still a long shot. “There’s lots of reasons this would never work,” Shah said.

While Mexico has been openly hostile to the wall, it remains to be seen whether the government would be more open to cooperating with the U.S. president if the wall also turned out to be a source of clean energy.

“Once CFE got wind of what Trump would do, they would say, even if this is a great deal for the Mexican people, ‘We’re not going to sign this contract on principle, because we don’t want Trump to fulfill his promise,’ ” Shah said.

Others argue there is still a compelling political and economic case to build a solar wall that would generate power for Mexico.

The power generated by the wall would go much further in Mexico. The solar panels could end up powering about 500,000 homes in the United States or “far more” across the border, according to solar experts Vasilis Fthenakis and Ken Zweibel.

“Even without buy-in from Mr. Trump, the Mexican president could pursue this wall on his own territory, with financing from private investors,” they write. “This would put a positive spin on Mr. Trump’s idea of a structure to divide the two countries. [President Enrique] Peña Nieto could invite his northern neighbors to take part in the initiative, or Mexico could simply reap the financial and environmental benefits for itself.”

Mexico is aiming to peak its emissions around 2026 and reduce its emissions intensity by about 40 percent from 2013 to 2030.

Trump apparently told legislators that they could talk about the wall as long as they said it was his idea.

Aside from the big economic and technical hurdles to building a massive solar wall, there’s no indication to suggest that the president has considered the domestic political consequences of his decision.

Knocking coal plants offline to make way for a solar panel wall is unlikely to chime with the president’s “Trump digs coal” mantra.

Given China’s global domination of the solar-panel-making industry, it might be hard for Trump to defend himself against the charge that building a wall out of made-in-China solar panels would be a gift to an economic competitor – let alone an energy boost to Mexico.