Donald Trump has suggested that the US border wall with Mexico could "pay for itself" through solar energy.

The US president suggested to Republican senators this week that if solar panels were installed on the wall they would generate enough energy to fund his controversial building project.

At a rally in Iowa on Wednesday (21 June), Trump told supporters that he had come up with the idea of installing solar panels on the wall himself.

"Solar wall, panels, beautiful. I mean actually think of it, the higher it goes the more valuable it is. Pretty good imagination, right? Good? My idea," he said.

"We're thinking of something that's unique, we're talking about the southern border, lots of sun, lots of heat. We're thinking about building the wall as a solar wall, so it creates energy and pays for itself. And this way, Mexico will have to pay much less money, and that's good, right?" he told the crowd.

Building a wall along the US-Mexico border was one of Trump's key campaign pledges. The billionaire said it was a necessary measure to stop illegal immigration and drug trafficking. He has always insisted that Mexico will pay for the wall, but Mexican President Enrique Peña Nieto has strongly dismissed this proposal.

Trump's proposed 40-foot tall and 1,300 mile long border wall would cost an estimated $21.6bn (£17bn) to build, according to a US Department of Homeland Security report seen by Reuters in February.

During the presidential campaign, Trump said that the wall would cost around $12bn to build. Republican House Speaker Paul Ryan and Senate Majority Leader Mitch McConnell estimated that the total cost would amount to around $15bn.

To cover the entire wall with 21-square-foot solar panels, around 13,358,136 panels would be needed, Gordon Johnson, managing director of Axiom Capital Management, told Bloomberg this week. He said the extra equipment and construction costs could add around $7.6bn to the total estimate.

If constructed, the border wall with solar panels could generate an annual profit of $221m, Johnson estimated. Without factoring in rising inflation, he said it would take the US government 125 years to pay the American taxpayer back the $28bn total cost of the wall.