Trivia question: How big is the U.S. supply of animal fat?

While driving to work or eating breakfast today, you probably jerked up and thought, "How much inedible tallow gets produced in the U.S. every year?"

Alternative energy is one of the primary topics of the day. Hobbyists now use old vegetable oil to run their modified Mercedes diesel cars, but that's a drop in the bucket of the industry. A number of meat companies and start-ups now want to methodically collect fat and used oil and turn it into millions of gallons of fuel. (Most biodiesel refiners now concentrate on fresh oil.)

So how much waste product is out there? We asked Vernon Eidman, professor of applied economics at the University of Minnesota and an expert on biofuels.

"I estimate that we produce 2.7 billion pounds of yellow and brown grease per year. We produce 1.1 billion pounds of lard, 1.9 billion pounds of edible tallow, 3.7 billion pounds of inedible tallow and 4.2 billion pounds of poultry fat per year," he wrote.

Yum.

"If we can collect about 60 percent of the yellow grease, that could add 200 million gallons of biodiesel per year. If we can convert half of the inedible tallow and poultry fat, that could add another 500 million gallons."

The fuel made from animal fat, Eidman emphasized, is more suited for heating diesel than running cars or trucks.

That would still be a drop in the overall diesel bucket. The U.S. consumes about 62 billion gallons of diesel a year, according to Martin Tobias at Imperium Renewables.

Still, it's a start. Only 225 million gallons of biodiesel got made in the U.S. last year, said Eidman. Worldwide, 5.5 billion gallons of biodiesel got produced, Eidman said.