Starting a solo career is something that musicians usually do when their old bands are running out of steam. But in the case of Nate Ruess, he stepped out on his own when his band, fun., were at their world-beating peak.

The New York indie-pop outfit’s second album, “Some Nights” (released in 2012), clocked up the kind of numbers and accolades rarely seen in the music industry these days: It sold 1.6 million copies in the US alone; the singles “We Are Young” (featuring Janelle Monáe) and “Some Nights” hit No. 1 and No. 3 on the Billboard Hot 100 Chart, respectively; and at the 2013 Grammys, the band walked away with two awards.

“We made a lot of people a lot of money,” Ruess explains to The Post. But the fun. gravy train lost a couple of key passengers. First, guitarist Jack Antonoff launched his acclaimed band Bleachers last year, and now Ruess is releasing his debut solo album, “Grand Romantic,” on Tuesday.

The sight of a red-hot band putting things on ice seems insane, and rumors of disquiet in the band (which also includes multi-instrumentalist Andrew Dost) naturally followed. The trio have been at pains to explain that they are not breaking up, even stating as much in a lengthy Facebook post earlier this year.

It’s true that fun. might not be done, but since Antonoff is still touring hard with Bleachers and Ruess is planning to concentrate on his album, they are likely to remain inactive for a while. Ruess concedes that, creatively at least, some space was probably needed.

“I can be a beast in the studio with my songs,” he admits. “So it’s good when people step out of that, because sometimes it can be constricting — and Jack killed it [with Bleachers]. When you’re in a band, you’re doing a lot of sharing. I wasn’t ready to do another album, because, right now, I’m enjoying my time alone.”

It’s something that comes through on the bold, baroque pop of “Grand Romantic,” much of which is thematically inspired by his relationship with fashion designer Charlotte Ronson (twin sister of Samantha Ronson and younger sibling of “Uptown Funk” producer/musician Mark Ronson).

“On the albums I’m used to writing, there’s usually a little more sulking,” laughs Ruess. “But on this one, I felt like celebrating. I feel like I have learned a lot more in the last year than I have in a long time. I’m more open to being vulnerable than I ever have been.”

The other big love in his life is New York City. The singer moved back to town last year after a stint in Los Angeles to be closer to Ronson, and admits that his adoration of the city fed into the album, too.

“New York is the ‘Grand Romantic,’ ” he says. “It’s the best place to fall in love, and I’m so thankful that I get to live here. I keep hearing that people are moving to LA all the time. That’s fine, because it means there’s more New York for me!”