President Trump's proposal to put solar panels on a wall that would span the Mexican border with the U.S. isn’t a bright idea. Rather, solar experts and analysts say it’s daft.

Trump suggested during a speech at a rally in Cedar Rapids, Iowa, last week that by installing solar panels the wall would pay for itself. The cost to build the famously controversial wall, which amounts to billions of dollars, is a sticking point for many taxpayers, not to mention Congress.

A detailed analysis by Bloomberg energy columnist Liam Denning, however, shows that if the wall were “somehow completed by 2020, it would have paid for itself by the year ... 2168.” That analysis is based on building cost and per-kilowatt-hour revenue rate estimates.

At “Climate Day LA” on June 27, Los Angeles Mayor Eric Garcetti spoke of the benefits of renewable energy to help mitigate climate change. Solar experts on hand were asked by Private Wealth about the idea for solar panels being installed on the proposed border wall.

“It ignores the cost of transmission lines,” said one leading solar expert. He explained that without utility lines, power transmission lines would have to be laid, and they are massively expensive. It’s a valid point that any homebuilder can relate to. Bringing power and water online—even just yards from lot to street—is arduous and costly. Imagine a lot nearly 2,000 miles long covering rough and remote terrain? Bringing power online and transmitting it would be exorbitant.

Cost benefits aside, simple physics may be even a bigger obstacle to such a plan. Walls are vertical. Solar panels are meant to lie horizontal. That’s why desert solar farms take up so much space.

Bloomberg’s Denning says the wall would have to slope at sixty degrees (on the U.S. side) to generate enough width for panels. The wall could only slope at a less-solar-friendly 20 percent on the Mexican side to make it a formidable deterrent to those wishing to enter the United States illegally.

The final point Trump made, about the taller the wall, the more energy it would reap (presumably meaning the panels would be closer to the sun), doesn’t hold promise either: The sun is 93 million miles from Earth. Even a height of a few hundred feet, let alone the proposed 30-foot height of Trump's wall, would make a negligible difference in the amount of captured solar radiation.

Thus far, the beneficiaries of the solar wall scheme are, ironically, the very people Trump has portrayed as the nation’s arch business enemy: Chinese business owners. The stocks of Chinese solar panel companies traded up after Trump unveiled his idea.

A further irony is the Trump administration's embrace of solar energy, which deeply tied to something of which Trump is highly skeptical—climate change. 

“My idea,” Trump boasted about the solar panel plan at the Iowa rally. By most accounts, the idea isn’t a good one.