Since October 2014, SolarCity Corp. has tried to lure individual investors to the solar-power business by pitching $214 million of what it calls “solar bonds” through the company’s website.

The biggest buyer by far, though, was rocket maker Space Exploration Technologies Inc., including $90 million of $105 million sold last month.

The bonds were an “excellent investment,” billionaire entrepreneurElon Musk said in an interview. And he knows more about the companies than anyone. Mr. Musk is their largest shareholder, the chairman of SolarCity and chief executive of SpaceX.

Mr. Musk, 44 years old, has built a business empire like no other in the world, fueled by his voracious appetite for risk and unyielding confidence. The three companies he leads—SolarCity, SpaceX and car maker Tesla Motors Inc. —are worth nearly $50 billion combined.

He has transformed every industry he has touched, speaking without irony about retiring to Mars after SpaceX successfully launches rockets there.