President Donald Trump outlined his vision for a wall along the U.S.-Mexico border Wednesday night, telling reporters on Air Force One that he is serious about installing solar panels to help pay for the multi-billion dollar project.

When asked if he was joking when he first floated the idea for a solar-powered wall last month, the president was emphatic.

“No, not joking, no. There is a chance that we can do a solar wall,” Trump said. “We have major companies looking at that. Look, there’s no better place for solar than the Mexico border — the southern border.”

Trump has repeatedly insisted that Mexico will eventually pay for the border wall, but he has conceded that other funding streams will be necessary until a deal is worked out with the Mexican government. Although many Republican lawmakers have been reluctant to commit funds, the House Committee on Appropriations announced Tuesday that its FY2018 appropriations bill for the Department of Homeland Security will allocate $1.6 billion for the wall. 

House Democrats will almost certainly fight the inclusion of wall funds as a part of DHS discretionary spending, which could lead to a protracted stalemate over an FY2018 budget deal.

In the meantime, Customs and Border Protection (CBP) is evaluating wall designs submitted by interested companies in the spring. Prototype construction was originally scheduled to begin by June 1, but CBP recently announced that testing is postponed until late summer.

En route to Paris Wednesday night, Trump offered his own suggestions for wall designs.

“One of the things with the wall is you need transparency,” he said. “You have to be able to see through it.”

“As horrible as it sounds, when they throw the large sacks of drugs over, and if you have people on the other side of the wall, you don’t see them — they hit you in the head with 60 pounds of stuff? It’s over,” Trump added. “As crazy as that sounds, you need transparency through that wall.”

During the campaign, Trump envisioned a wall across the entire 1,954-mile border with Mexico, but he told reporters that he’s been convinced there are several stretches of land that don’t need a manmade wall because of natural barriers.

“But you’ll need anywhere from 700 to 900 miles,” he said.

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