Donald Trump’s plan to build a solar border wall, explained with math

Last week, President Donald Trump insisted that he really isn’t joking about putting solar panels on the border wall he’s promised to build between the United States and Mexico.

“There is a chance that we can do a solar wall. We have major companies looking at that. Look, there’s no better place for solar than the Mexico border — the southern border,” he said. “And there is a very good chance we can do a solar wall, which would actually look good.”

When Trump first pitched the idea of a solar wall at a meeting with Republican Congress members at the White House in June, he claimed that the idea was his (it wasn’t) and that the energy generated by the panels could cover the cost of the wall.

Now that it seems he really wants solar panels on there, we decided to do the math on this possibility of the wall “paying for itself.” That would, theoretically, help him sell the plan to fiscal conservatives, among others.

As Vox’s Dara Lind and Tara Golshan explained in May, Trump faces a multitude of obstacles in building more border wall (there are already 654 miles of fence and wall down there). Chief among them is convincing Congress to pay for it. While Trump maintains that he will somehow get Mexico to eventually pay for the wall, American taxpayers will have to foot the bill initially.

On July 11, Republicans in Congress released a bill that would allocate $1.6 billion to the Department of Homeland Security to begin construction of the physical barrier. But that’s just a fraction of the $21.6 billion the DHS estimated in February would be needed to build a wall along the 1,250 miles of unfenced border. (Trump told reporters on July 13 that he now intends to build only 700 to 900 miles of “see-through” wall, but that would still cost roughly $12 billion to $15.5 billion, according to the February DHS estimates.)

The idea to put solar panels on the wall seems to have originated in a Huffington Post article last December, but it gained greater traction after Gleason Partners submitted a prototype design in April to DHS’s Customs and Border Protection.

In June, CPB announced plans to build four to eight different wall prototypes this year in San Diego. (It’s not yet clear if Gleason’s solar border wall will be one of those prototypes.)

But a back-of-the-envelope calculation shows that solar panels wouldn’t come close to covering the cost for the 700 to 900 miles of wall Trump wants to build.

To simplify things, I’ll give Trump the benefit of the doubt with a best-case scenario.

The average commercial solar panel is about 39 by 77 inches, and utility-scale solar arrays typically hold two rows of panels vertically. In a scenario with the maximum number of panels installed along the top of the wall (with no horizontal space between them), a 900-mile border would at most be able to accommodate 2,924,102 panels across its width.

The typical commercial solar panel has a capacity of up to 350 watts, so these nearly 3 million panels could make up a system of just over 1000 megawatts. But limited daylight hours and adverse weather conditions, such as extreme heat or excessive cloud shading, can cause solar panels to operate at less than maximum capacity. The average output capacity factor of solar panels in the US is around 20 percent.

Nevertheless, solar technology continues to improve, and the southwestern part of the United States is sunnier than average. Optimistically, let’s assume solar panels atop a border wall would operate at an annual 30 percent capacity factor. The nearly 3 million panels would then generate upward of 2.7 terawatt-hours of electrical power per year.

Let’s also say, for Trump’s sake, that Mexico’s electric utility would buy all the energy generated and pay for all the transmission infrastructure. The average retail price of electricity in the border states is about 11 cents per kilowatt-hour. Assuming that the energy generated by the border wall is sold at that retail rate (it would more likely be sold on the cheaper wholesale market and in Mexico, but we’re giving Trump the best-case scenario here), the annual revenue would be about $300 million per year.

But with standard utility-scale solar installation costs as low as $1 per watt, the nearly 3 million panels would cost at least $1 billion. Even with the generous revenue estimate, it would take four years just to recuperate the installation costs of the panels.

Now, remember that 700 to 900 miles of new wall would cost approximately $12 billion to $15.5 billion to build. And this and the existing fencing would also need to be maintained. In 2009, the Government Accountability Office estimated that the current fencing along the border would need $6.5 billion for upkeep and repairs over the next 20 years (an annual average of $325 million). A much longer wall would cost even more than that, not to mention the additional operations and maintenance costs of the solar panels, which need to be cleaned, repaired, and replaced regularly.

So when all is said and done, it looks like the solar panels, estimated to bring in at most $300 million a year, would hardly make a dent in the wall’s costs.

According to KCET’s Chris Clarke, who also tried to estimate the incredibly high cost of a solar wall, a version like the one Gleason Partners proposed would also be very technically difficult to build.

For starters, a wall is not an ideal frame for solar panels. Solar panels traditionally lay at a slight angle off the horizontal, such as on roofs or in large solar farms. Making the vertical part of a wall out of solar panels wouldn’t make sense, since, after all, they must face the sun to convert its energy.

To account for this, Gleason, the prototype designer, proposed angling two sets of solar panels off the wall — one on the top, the other jutting from the middle to the ground, as pictured below.

However, solar panels in the Northern Hemisphere should point south to maximize time facing the sun. But in the case of a border wall, that means the most valuable parts of the wall, the solar arrays, would have to be displayed, unprotected, on the Mexican side. This would leave them open to almost certain vandalism and theft.

To maximize efficiency for capturing sunlight, a border wall could arrange panels from the top to the ground at an approximately 30-degree angle off the horizontal. But as Clarke points out, a structure like that would be easy to climb and more of a border ramp than a border wall, certainly not fulfilling its intended purpose. (It would also be difficult for a wall made of solar panels to be transparent. Donald Trump has said it should be to prevent sacks of drugs from being tossed over and hitting unassuming Americans.)

Trump’s border wall plan is already really unpopular. And putting solar panels on the border wall shouldn’t make fiscal conservatives any more excited about this extremely expensive infrastructure project. And according to recent polling by Echelon Insights, it doesn’t. The solar panel idea has also resulted in only slightly less opposition by Democrats, according to the same polling.

The green energy component has done little to mitigate concerns environmentalists have about the wall either. As Brett Hartl of the Center for Biological Diversity affirmed in a statement in June, “An ecological disaster with solar panels on top is still an ecological disaster. With solar panels on top.”