Solar stocks flew on Thursday after President Donald Trump talked up the idea of a border wall providing solar energy in a speech to a rally in Cedar Rapids, Iowa. (See also: Solar Energy to Cost Less Than Coal by 2021.)

The Republican president told fans that the design team is “thinking of something that’s unique,” on the southern border, where there is “lots of sun” and “lots of heat.” Trump suggested that a solar wall would create energy that pays for itself. “And this way, Mexico will have to pay much less money, and that’s good, right?” added Trump. “My idea.”

Fact Check for Border Wall

Analysts at Bloomberg indicate that the solar wall would in fact not pay for itself, assuming a 1,933-mile length, 30-feet height, 60-degree angle on the south side and a more solar-friendly 20-degree angle on the north side. Bloomberg energy columnist Liam Denning modeled the wall’s annual revenue at $1.52 billion with estimated construction costs of $40 billion. Taking into account the cost of borrowing, maintenance and construction, Denning says Trump’s wall could possibly pay for itself by 2168, not yet taking into account the diminishing value of money over time.  

Whether the cost of the proposed border wall would be totally covered by solar energy or not, media reports indicate that solar panels are indeed among the proposals submitted to the Department of Homeland Security by companies interested in the project.

Shares of Tempe, Ariz.-based First Solar Inc. (FSLR) have gained more than 10% since Wednesday to $39.29 on Friday morning, while commercial and residential sector leader SunPower Corp. (SPWR) jumped more than 18% to $8.88. The news comes the same week as investment bank Goldman Sachs upgraded solar players Vivint Solar (VSLR) and Sunrun Inc. (RUN) to Buy on exoected M&A activity in the segment. (See also: 2017: A Turning Point for the Solar Industry.)

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