While speaking to reporters on Air Force One late Wednesday, Trump said he has “major companies looking” at the option.

“Look, there’s no better place for solar than the Mexico border — the southern border,” Trump said. “And there is a very good chance we can do a solar wall, which would actually look good. But there is a very good chance we could do a solar wall.”

Trump also said that he thinks the wall should be transparent as a security measure.

“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 on the head with 60 pounds of stuff? It’s over,” he said.

Trump’s comments came during an off-the-record briefing with reporters that was made on-the-record on Thursday.

The border between the U.S. and Mexico is nearly 2,000 miles long. But Trump estimated that only 700 to 900 miles of new wall would need to be built, as some portions are blocked by natural barriers like mountains and rivers, while other sections have existing walls.

A border wall between the U.S. and Mexico was a major Trump campaign promise during the 2016 presidential election. Trump first mentioned the possibility of adding solar panels in June, which led to a temporary boost in stocks for solar power companies. The idea was previously discussed in a Wall Street Journal op-ed in March.