The Mesquite 3 solar array in Arizona. (Sempra Energy)

On Friday in Maricopa County, Ariz., the U.S. government will hit a clean energy milestone: What officials are calling the largest procurement ever of renewable energy by the federal government, in this case from a desert solar array.

The new 150-megawatt, or million-watt, Mesquite 3 solar array is located in Arizona, but the electricity it generates will be sent to California’s electric grid and will power roughly one-third of the electricity needs of 14 naval installations in the state, including San Diego’s naval base and the Marines’ Twentynine Palms and Camp Pendleton.

“Today we’re going to throw a switch and start getting those electrons flowing to our 14 bases,” said Dennis McGinn, the assistant secretary of the Navy for energy, installations and environment, who spoke from Arizona where he was on site for the opening ceremony for the plant.

“It’s going to be reliable, it’s going to be cheaper than what we’re paying for brown power, and it just diversifies our energy sources for these bases,” McGinn continued.

The deal will let the Navy buy solar power at a fixed price for 25 years from the array, which is owned by Sempra Energy. “To me, the essence of solar power is, you know what the price of the fuel is going to be for the next 25 years, or more,” McGinn said. “It’s predictable, it provides financial planning and energy planning stability to our calculation, and it’s part of our diversified energy portfolio.”

The Navy has distinguished itself as a leader in the clean energy space of late, also experimenting with biofuels for planes, ships and vehicles and even trying to subtly change officers’ behavior to make them more energy conscious, based on the idea that using less energy provides a tactical advantage — in some cases, it literally means the ability to fight for longer if you have to.

The move is being celebrated not only by the Navy, but also the Energy Department, which contends that the dramatic growth of large-scale solar plants in the Southwest is a direct result of major investments made by its Loan Programs Office as part of the stimulus legislation passed in the wake of the financial collapse in 2008-2009.

In 2010, the United States didn’t have a single utility scale solar photovoltaic farm, a new analysis from the Loan Programs Office notes. But $4.6 billion in loans from the Energy Department led to the installation of five such plants, capable of generating 1.5 gigawatts, or billion watts, of electricity capacity. One of them was the 170-megawatt Mesquite 1 solar plant, the predecessor to Mesquite 3.

“Before 2009, utilities had a total of just about 20 megawatts of PV, total,” Energy Secretary Ernest Moniz said in an interview Friday. “So then came this couple of years when the loan program kick-started, as you can see, with 1.5 gigawatts.”

That was the end of the federal program — but today, there are 45 more such large-scale projects, adding another roughly eight gigawatts of capacity, mostly in the Southwest still but also in states ranging from Minnesota to Georgia.

When the private sector took over. Moniz contends that was in part possible because the Energy Department program had been able to drive down prices. “That’s kind of a virtuous cycle of deployment plus innovation keep combining to drive cost down,” Moniz said. “So obviously, when there was no deployment, there was no benefit of going down the curve.”

The over 10 gigawatts of installed utility scale solar photovoltaic capacity in the United States today is just one part of the tremendous solar boom the country has seen. None of this takes into account more medium-sized arrays or individual rooftop solar installations. The Energy Department has also given loans for a different type of large-scale solar array, called concentrated solar power, many of which also have been built.

According to the Solar Energy Industries Association, there are 31.6 total gigawatts of solar photovoltaic capacity installed in the United States, capable of providing enough electricity for 6.2 million homes.

Moniz said that solar still only provides about 1 percent of total U.S. electricity capacity. In the future, he noted, as the grid adds more and more variable wind and solar assets, “you have to start balancing in the grid, and that can be through storage. It can also be by, let’s say, integration with fast-ramping natural gas.” But for now, he said, “the grid has plenty of capacity to absorb these variable inputs.”

And the fact that the Navy will now be powered by them in California pretty much cements an increasingly common argument: If climate change is a national security threat, clean energy is part of the answer.

Correction: This story previously stated that there had been 40 more large-scale utility projects since the end of the federal loan program. The actual number was 45.