It isn’t really Christmas without “A Christmas Carol.” Since 1843, when Charles Dickens published what he called his “little Christmas book,” it’s never gone out of print — selling countless copies and spawning thousands of plays, readings, TV shows and movies starring everyone from Alastair Sim, George C. Scott and Christopher Plummer, to Jim Carrey, Scrooge McDuck and the Muppets.

This year marks the 150th anniversary of Dickens’ reading tour in America, which seems to have renewed interest not only in “A Christmas Carol” itself — but what prompted him to write it. The new film “The Man Who Invented Christmas,” an exhibition at the Morgan Library & Museum and even a novel all seem to come to the same ironic conclusion: that Dickens might never have written his plea for charity if he hadn’t been desperate to make money.

“He was in debt up to his eyeballs,” Samantha Silva, author of the new novel, “Mr. Dickens and His Carol” (Flatiron Books) told The Post. His last works had flopped: No one cared for “American Notes for General Circulation” (based on an unhappy, 1842 trip to the States), and his latest serialized novel, “Martin Chuzzlewit,” was such a dud, Dickens’ publishers docked £50 from his salary each month. Meanwhile, his wife, Catherine, was expecting their fifth child and they’d just leased a grand London home, which Dickens refurbished.

“He was living well beyond his means,” noted Declan Kiely, who curated the “Charles Dickens and the Spirit of Christmas” show at the Morgan. “Nothing would do except marble mantels and paneled mahogany doors.” The writer also loved to entertain, never stinting on the whisky, rum and brandy.

But Dickens’ generosity extended far beyond his own needs. “Though he fretted about money, he supported many charities,” Silva noted. “It was said that he rarely passed a beggar on the street without pressing a coin in his hand.”

Not only did he have his own debts to pay, but he was responsible for his father’s. It was hardly the first time. Decades before, his parents and younger brothers were carted off to debtors’ jail. Only 12-year-old Charles stayed behind, working in a blacking factory to pay their bills. Those long miserable months, Silva said, “made him a passionate advocate of the poor and defenseless.”

After he’d written his way out of poverty, his father and brothers borrowed money in his name. Feeling as squeezed as Scrooge — and just as contemptuous of the relatives he called his “blood-petitioners” — Dickens decided to pen a quick hit: a Christmas book.

As he told friends, the tale of Scrooge’s journey to redemption took shape during his nightly walks through London. “It was his custom to walk 15, maybe 20 miles,” Silva said.

“There’d be wonderful streets, where people with money lived, and three blocks away squalor. He’d look in people’s windows and see families like the Cratchits gathered around a table or hearth.”

Dickens recorded everything he saw, smelled and heard on scraps of paper, Silva said: “Everything was material to him.”

The names of his characters came, as they often did, from the people he met and the sounds he enjoyed — like the melding of “gouge” and “screw” that make up Scrooge. Silva believes Scrooge’s first name came from a group called the Ebenezer Temperance Society: “Dickens couldn’t abide hypocrites who wanted to save souls but take away what little pleasure they had.”

The character itself, Kiely says, is based on an earlier creation: Gabriel Grub, the nasty, child-beating sexton of “The Pickwick Papers.” But there’s a little of Dickens’ own, cash-pressed self in there, too, Kiely says: “A philanthropist by nature, he was an immensely imaginative person who could ask himself, ‘What if I were the opposite of this?’ He loved to look into the dark side of London life, and also the dark side of his own nature.”

In the original manuscript — the gem of the Morgan’s collection, on display now — Tiny Tim was initially called Tiny Fred. Kiely said he was based on Henry Burnett, the sickly son of Dickens’ sister, Fanny. The real boy died in childhood, a few years after Tiny Tim entered the world.

Working steadily, Dickens started writing “Carol” in mid-October and finished it six weeks later. Since deciding to publish it himself, he was determined it be beautiful.

“He wanted gilt on the cover, gilded paper edges and four hand-colored illustrations — for 6,000 copies,” Kiely said. “And it had to be done in 16 days.”

Dickens gave his printer the manuscript on Dec. 3. By Christmas Eve, all 6,000 copies had been sold at 5 shillings each, which Kiely notes was one-third of Bob Cratchit’s measly, 15-shilling-a-week salary.

Though Dickens hoped to make a thousand pounds from “Carol,” his production costs were so high, he netted only a quarter of that. So dire were his finances that the Dickens family decamped for a while to Italy, where the living was cheaper.

But “A Christmas Carol,” widely embraced from the start, gradually made money. Everyone wanted to hear the tale of Scrooge’s change of heart, especially read by the author himself. And so, Dickens, whom Silva calls “a frustrated actor,” started touring. A wood engraving in the Dec. 28, 1867, Harper’s Weekly showed people “laying out blankets in the snow in front of Manhattan’s Steinway Hall,” Kiely said, waiting to buy a $2 ticket. He was, as Silva put it, “a literary rock star.”

His books and readings helped Dickens provide not only for his family, but the unfortunate around him. “Distressed,” Kiely said, by the number of teenage prostitutes on London’s mean streets, Dickens helped set up a halfway house for them, designing a course of study (and uniforms) to train them to become governesses and teachers.

But even a man with “a heart as big as the world,” as Silva puts it, was still a man, and a flawed one at that. In what Kiely calls “a classic midlife crisis,” the 46-year-old Dickens had an affair with an 18-year-old actress, Nellie Turner. His wife, who’d grown fat and dull after bearing 10 children, was kicked out of their home, while Dickens enlisted her sister’s help in raising their children.

He died at 58, having delivered, in “A Christmas Carol,” what Silva calls “a sledgehammer blow against the miserly rich.”

In reminding us to honor this season of rebirth and charity — “and try to keep it all the year” — Dickens gave the world a present, not only in years past, but now and in the future.