Energy Secretary Ernest Moniz, left, and Vice President Joe Biden tour the headquarters of energy company PECO in Philadelphia on April 21. (Matt Rourke/AP)

The energy secretary’s job title has always been a bit of a misnomer. Nearly two-thirds of the department’s budget has nothing to do with energy. Instead, it is devoted to taking care of the nation’s nuclear weapons stockpile — and cleaning up radioactive waste from old weapons development sites.

That nuclear expertise has catapulted Energy Secretary Ernest Moniz, a physicist from MIT, from a Cabinet backwater to center stage in the negotiations over Iran’s nuclear program. An unlikely breakout star, Moniz sports a long wavy mop of mostly white hair that has been compared to George Washington’s locks.

The energy secretary is providing a knowledgable voice that the Obama administration hopes can reassure nervous members of Congress as they weigh the deal.

Moniz is someone who “can take complex issues and make the lay person understand them,” said Carol Browner, who was the top White House adviser on energy and climate issues during the first Obama term. “His ability to do that with members of Congress will be important.”

A Republican congressional staffer said: “He’s not a policy wonk sitting around drawing two-by-two matrix models on a whiteboard. He’s a physicist talking about what the technical capacities are. Moniz is able to speak a different language and translate that into better arguments than we’ve seen so far.”

Moniz has suddenly found himself to be one of the most in-demand Obama Cabinet secretaries. Since returning from the Iran negotiations in Lausanne, Switzerland, on April 3, he has traveled to Panama with President Obama on Air Force One; briefed members of Congress on the Hill along with Secretary of State John F. Kerry and Treasury Secretary Jack Lew; joined Vice President Biden in Philadelphia to talk about energy infrastructure; and met, as a member of the U.S.-Iraq Higher Coordinating Committee, with visiting Iraqi Prime Minister Haider al-Abadi. He would have traveled to Beijing with the commerce secretary but couldn’t squeeze it in.

Moniz has been to Washington before as undersecretary for energy and associate director of the White House office of science and technology policy under President Bill Clinton. And he enjoys and is adept at dealing with Congress, indeed perhaps more so than the president and many other members of the administration.

Faced with proposed legislation that would have forced the Energy Department to speed up decisions on applications to export liquefied natural gas, Moniz called the bill’s sponsor, Sen. John Hoeven (R-N.D.), and asked him to hold the bill so the department could work with the senator to craft a mutually acceptable version. In the end, the bill set the same time limit but started the clock at a different point in the regulatory process. It left both the senator and the administration satisfied; committee members from both parties voted in favor of it.

“I appreciate Secretary Moniz reaching out to work with me to streamline the process,” Hoeven said in a statement.

In August, Moniz trekked to Alaska with Sen. Lisa Murkowski (R), before she became chairman of the Senate Energy and Natural Resources Committee. And he keeps in touch with Sen. Edward J. Markey (D-Mass.), who attended events with Moniz at MIT before Moniz became energy secretary.

Moniz took over from another physicist, Steven Chu, who won the Nobel Prize for using lasers to freeze and trap atoms. Obama turned to Chu, too, for technical advice to figure out how to stop the massive BP oil spill in the Gulf of Mexico.

But Chu, previously a professor at Stanford and the University of California at Berkeley, as well as director of the Lawrence Berkeley National Laboratory, often had strained relations with Congress. Lacking ties to the Hill, he found himself a frequent target when a solar manufacturer, which had received a ­half-billion-dollar loan guarantee from the Energy Department, went bankrupt; lawmakers raked through the department’s internal e-mails for evidence of political favoritism.

“I’d characterize him as a much better version of Chu, that he has scientific credibility but with a better attitude and political instincts and more experience in D.C.,” said one of President George W. Bush’s top Energy Department officials. “Therefore, he does much better with Congress and does a much better job of running the department and engaging employees.”

Moniz also has paid attention to internal matters at the department, which he long thought had been mismanaged. He created a new post — undersecretary for management. He also has recruited people from a wide variety of backgrounds, including a White House lawyer, Duke Energy’s chief technology officer, the head of the Union of Concerned Scientists, the ­arms-control expert from the National Security Council, the head of an energy institute at Stanford and the retired commander of the Air Force Global Strike Command, with control of U.S. intercontinental ballistic missiles and bombers.

Moniz was dispatched to Switzerland when Iran sent Ali Akbar Salehi, the head of the Atomic Energy Organization of Iran, to join the talks. Salehi did graduate work on fast neutron reactors at MIT. He and Moniz did not meet there, but the two spoke the same technical language and were able to negotiate fluently.

The two negotiated in small groups, but also spent many hours alone in face-to-face meetings. Doing so allowed other negotiators to focus on separate issues, such as sanctions relief. Salehi had at times been a critic of the negotiations, so his participation was seen as an effort to finally hammer out acceptable terms.

“The role was fundamentally to resolve these technical dimensions,” Moniz said back in Washington. “In doing so, we can uncouple that part from the political dimensions of the agreement.”

Although Moniz joined the Iran talks only recently, the State Department had already been drawing on expertise at the nation’s eight national laboratories, which fall under the Energy Department.

Moniz, too, has relied on teams of these experts, asking them to check calculations about the “breakout time” Iran would need to accumulate material for a nuclear weapon and using “red teams” in the United States to try to figure out whether the terms of the agreement could be circumvented.

The lab experts often started working when the negotiators finished for the day, and they had to hustle to get an analysis done by the time negotiators woke up. One lab expert said he had three 22-hour days in a row, staying up past 4 a.m. and returning by 8:30 a.m. to be on hand for any new questions.

“Basically, our job was to address issues with major technical dimensions,” Moniz said. “What would an Iranian nuclear program look like over quite a few years to meet our requirements of confidence in the peaceful nature of the program, to be able to identify quickly if in fact activity were not in the bounds of the agreement and to provide enough time in that eventuality that we and our partners could respond appropriately?”

Moniz says he works easily with Kerry, whom he has known for more than a decade. Moniz was part of a group of energy experts who gave advice to Kerry’s presidential campaign in 2004.

It was just one way in which Moniz stayed engaged with Washington. At MIT, he co-chaired projects on the future of nuclear power and the nuclear fuel cycle. During the first Obama term, he was a member of the Blue Ribbon Commission on how to dispose of nuclear waste.

Before his nomination, he devoted himself to the “all of the above” strategy for energy that Obama has embraced. In a voluminous written and spoken record, Moniz supported nuclear power, research into carbon capture and storage for coal, renewable energy and shale gas produced by hydraulic fracturing.

All that has fed into this potentially historic moment on Iran.

“Now, if we are able to obtain a final deal that comports with the political agreement — and I say ‘if’ because that’s not yet final — then I’m absolutely positive that that is the best way to prevent Iran from getting a nuclear weapon,” Obama said in a briefing. “And that’s not my opinion, that’s the opinion of people like Ernie Moniz, my secretary of energy, who is a physicist from MIT and actually knows something about this stuff. That’s the opinion of a whole bunch of nuclear experts who examined the deal.”