Cargo trucks gulp about 40 percent of the fuel pumped in the U.S. While most consumer attention focuses on improving the fuel economy of consumer vehicles, a major opportunity goes rumbling by. “Folks do not realize that the fuel use of even a small truck is equal to many, many cars,” says Bill Van Amburg, senior vice president of Calstart, a clean transportation technology nonprofit, and director of the Hybrid Truck Users Forum. “A utility truck as a hybrid would reduce more petroleum than nine Priuses.”

Some 1,300 commercial hybrids on the road today get up to twice the fuel efficiency of their conventional counterparts. But these traditional hybrids are inherently limited. They make more efficient use of petroleum-based fuel by capturing some of the energy lost during braking.

Plug-in hybrids, on the other hand, draw energy from the grid. They can drive for miles—in many cases, an entire day’s route—without using any fossil fuel at all. This shifts energy demand away from petroleum and toward grid-based sources. (Last year zero-carbon renewables and nuclear supplied 30 percent of all electric power in the U.S.)

In many ways, plug-in hybrid technology makes more sense for delivery trucks than for consumer sedans. A cargo truck runs a short daily route that includes many stops to aid in regenerative braking. Most of the U.S. Postal Service’s 200,000-plus mail trucks, for example, travel fewer than 20 miles a day. In addition, fleet vehicles return nightly to storage lots that have ready access to the 120- or 240-volt outlets required to charge them.

The Department of Energy recently launched the nation’s largest commercial plug-in hybrid program, a $45.4-million project to get 378 medium-duty vehicles on the road in early 2011. The trucks, which will go to 50 municipal and utility fleets, will feature a power system from Eaton, a large manufacturer of electrical components, on a Ford F-550 chassis. (For its part, Ford will wait for the market to prove itself before designing its own commercial plug-ins.) “These are going to start breaking free in 2011,” says Paul Scott, president of the Electric Vehicle Association of Southern California.

Start-up company Bright Automotive has a more ambitious plan. It aims to replace at least 50,000 trucks with plug-in hybrids by 2014. Bright’s IDEA prototype (above right) travels 40 miles on battery power before switching to a four-cylinder engine that gets 40 miles to the gallon. The streamlined aluminum body has the payload of a postal truck yet is far more aerodynamic. The truck weighs as much as a midsize sedan.

John E. Waters, Bright Automotive’s founder and the former developer of the battery system for General Motors’s groundbreaking EV1 electric car, says that each IDEA would save 1,500 gallons of fuel and 16 tons of carbon dioxide emissions a year over a standard utility truck. Waters says he is ready to begin assembly in his U.S. plant once a pending $450-million federal loan comes through.

Despite the appeal of the carbon savings, the fleet owners who are the trucks’ primary customers have more practical considerations. Bright’s executives are coy about the IDEA’s eventual price tag but assert that a customer with 2,000 trucks driving 80 miles a day five days a week could save $7.2 million a year. Right now that is probably not enough to justify large-scale purchases without additional rebates—or a price on carbon. Van Amburg estimates that going hybrid currently adds $30,000 to $50,000 in upfront costs per vehicle, although that figure should come down as production volumes increase.

Improved battery technology will also help. Today the IDEA’s 13-kilowatt-hour lithium-ion battery pack accounts for nearly a quarter of the vehicle’s total cost. Much of the research being done for the batteries going into the Chevy Volt and other consumer plug-ins should also be applicable to commercial batteries. “For all the good we all want to do,” says David Lauzun, Bright’s vice president of product development, “these vehicles will not take over the world until it becomes the economic choice—‘I have to have them because it saves me money.’”