(CNN) -- The Defense Department has a grand vision for the U.S. military's energy future, including "green"-powered fleets, jets and trucks. But members of Congress are hung up on the dollar signs that come with going green.
Language in the House and Senate versions of the defense budget largely bans the use of alternative energy like biofuels, prohibiting the military from purchasing any alternative fuel that costs more than traditional fossil fuels like oil. The catch: Biofuels are always more expensive than oil, about four times more.
"To have the military, whose sole job is to defend this country, spending extra money simply on flying their airplanes with fuel that's available at a cheaper price, again on these restraints and the resource restraints that we find ourselves in, makes no sense to me," said Rep. Mike Conaway, R-Texas, who introduced the amendment.
The Defense Department says it needs to invest in diversifying the sources of energy that fuel almost every piece of military equipment. The biofuels are considered a "drop-in fuel," meaning no changes to equipment engines are necessary.
"As a major consumer, probably the world's single largest consumer, of liquid fuels, we have an interest in making sure we have fuels for the future as well," said Sharon E. Burke, assistant secretary of defense for operational energy plans and programs. "So we're opposed to any efforts that restrict our options in this area."
It's been an Obama administration priority to decrease the U.S. dependence on foreign oil. The Defense Department sees energy independence as a national security objective. Since August 2009, the Defense Department has spent $42 million to purchase 1.1 million gallons of biofuels for testing purposes.
"Many of those sources of which we are absolutely dependent are in volatile or potentially volatile places on Earth, and some of those oil suppliers probably don't have our best interests at heart," Navy Secretary Ray Mabus said at a Senate hearing. "We would never depend on those oil suppliers to build our ships, our aircraft, our ground equipment, but we give them a say in whether the ships sail, the aircraft fly or the ground vehicles operate, because we depend on them for fuel."
Last year, the Navy spent $12 million for 450,000 gallons of biofuel, the largest purchase of biofuel ever in the U.S. The service, which has been a leading force behind the military's green initiatives, plans on using that fuel to power a carrier strike group during a two-day demonstration this summer. A green fleet will be ready for deployment in 2016. What the congressional restriction would do to those plans is unclear.
Pentagon spokeswoman Lt. Col. Melinda Morgan said the Navy will move forward as planned but warned that if the provisions are enacted, "they could affect some of the Navy's biofuels goals and restrict DoD's ability to increase our resilience against potential supply disruptions and future price volatility of petroleum products."
"The Great Green Fleet doesn't have an environmental agenda," Mabus said. "It's about maintaining America's military and economic leadership across the globe in the 21st century."
Every time the price of a barrel of oil goes up by one dollar, it costs the military $130 million, according to the Pentagon.
"When anyone says we can't afford to invest in developing alternative sources of energy, my reply is, we can't afford not to," he said. "We can't afford to wait until price shocks or supply shocks leave us no alternative."
Mabus is not blind to the large price tag of $26 a gallon that comes with the green fuel but says simple economics can solve the issue.
"Alternative fuels can't become competitive with oil unless there's demand for them," he said. "But demand at commercial scale will never be possible unless there's a supply to meet that demand."
The Defense Department's purchase of small amounts of biofuel for research and development has dramatically reduced the price of biofuels, cutting the cost in half in two years, according to Mabus. And the Navy is investing $170 million in the production of advanced "drop-in" aviation and marine biofuels to kick-start the U.S. alternative energy sector.
"It's really about investment today for pay off tomorrow," said clean energy advocate Phyllis Cuttino of Pew Charitable Trusts. "How much did the first pair of night vision goggles cost us? A lot more probably than they cost now."
But Conaway says it's not the military's job to get an industry off the ground.
"Is it the federal government's responsibility to start that industry? And I would argue that no, it is the private sector's out there, that's great at doing these kinds of things," Conaway said. "It's got to be, Southwest Airlines and American Airlines gotta be buying that fuel. And when that happens? Great. The Department of the Navy can buy it as well."
With the amendment passing in both the House and Senate Armed Services Committees, Conaway expects the restriction to be a part of the final defense budget.