A key part of President Barack Obama's State of the Union speech was a promise to support clean energy, with the clear implication that it would save money.

"I'm proud to announce that the Department of Defense, the world's largest consumer of energy, will make one of the largest commitments to clean energy in history, with the Navy purchasing enough capacity to power a quarter of a million homes a year," Obama said Tuesday night. "Of course, the easiest way to save money is to waste less energy."

But the Navy's alternative energy program isn't necessarily about saving money, although it should do that, too.

Ray Mabus, the secretary of the Navy, who is a strong proponent of clean energy, talked with CNN's Barbara Starr about alternative energy last February.

"Yes. It's going to be cost-effective," Mabus said, but he added, the main purpose of of the Navy's alternative energy program is energy security.

"We have simply got to quit buying as much fuel from potentially volatile regions of the world," Mabus said.

"We wouldn't allow some of the places that we buy fossil fuels from to build our ships, to build our aircraft, to build our ground equipment, yet we give them a say on whether those ships sail, on whether those aircraft fly, on whether those vehicles run, because we buy fuel from them."

But while Mabus sees strategy, not saving money, as the prime objective of the Navy's alternative energy program, sometimes there are financial benefits.

"We already have one ship that has a hybrid drive, USS Makin Island," Mabus said. "It saved $2 million in that initial voyage. Over the lifetime of that ship, on today's oil prices, it'll save a quarter of a billion dollars in fuel."