It's been nearly a year since Elon Musk revealed his intention to electrify the world's roads with buses and trucks in addition to Tesla's passenger cars. He hasn't said much about the 18-wheeler—a proper unveil is set for September—except that it will use same motors as the upcoming Model 3, and that it would, of course, disrupt an industry that generates one quarter of US transportation-related greenhouse gas emissions.

If anyone deserves the benefit of the doubt, it's Musk. But unless he has a radical bit of battery technology squirreled away, even he may not be able to deliver a long-haul truck capable of displacing the diesel burners roaming America's highways. That's the key finding of a paper by a pair of Carnegie Mellon University researchers, who found a battery-powered semi would be limited to a 300-mile range, cost a fortune, and offer limited cargo capacities because of the weight and volume of the technology required to keep it moving down the road.

“The challenge is on par in difficulty level with electric airplanes,” said Venkat Viswanathan, who conducted the research with colleague Shashank Sripad. The peer-reviewed study, previewed to Wired, will be published in the American Chemical Society's ACS Energy Letters within a few weeks.

The researchers considered data on average loads carried by trucks, average distances traveled, and a slew of specific trucking-related physical parameters, such as drag coefficients, rolling resistance, air density, and overall powertrain efficiency. The kind of full-size tractor trailers Musk wants to make cover, on average, 300 to 600 miles a day.

Say the Tesla truck runs a lithium-ion battery system that generates 243 watt-hours per kilogram at the cell-level—a good bet based on the tech in Tesla’s current cars. To cover 600 miles without stopping to charge, the truck would need a 14 ton battery. A 900-mile battery would weigh about 22 tons. Based on current prices, those packs would cost between $290,000 and $450,000. A comparable diesel rig costs about $120,000, all-in.

Price isn't the only question: Federal rules limit trucks' gross weight to 40 tons. Considering the heft of the battery pack, plus things like the cab, trailer, and wheels, the researchers figure a 600-mile-ready Tesla truck could carry just nine tons of cargo. That's two-thirds the current average payload of 16 tons. Even the lower operating costs and fuel savings of electric propulsion may not balance out such a disadvantage in an industry where efficiency rules. Meanwhile, the time it would take to top off massive batteries would slow things down considerably, although charge time could correspond with the mandatory breaks long-haul truckers must take.

“Our paper suggests that using a bigger battery pack to achieve longer range maximum payload is unfeasible, given the energy density of current lithium-ion batteries,” Viswanathan says. “Three hundred to 350 miles is probably what the vehicle could be designed for. Beyond that, the battery would be both very heavy and very expensive.”

Granted, that kind of range works for short, repetitive routes—say shuttling between ports and distribution centers—where air pollution in populated areas is especially worrisome. Even Toyota is working on a zero-emissions, 200-mile-range hydrogen truck for that use case. But Musk doesn't do restraint—he's targeting the semis that criss-cross the country.

Tesla officials declined to divulge more details of their program. And of course it is possible that Tesla engineers have figured out a way of making this work because Musk is Musk and seems to thrive on proving people wrong. Viswanathan estimates a next-generation “beyond lithium-ion battery pack” could provide a 600-mile range and cost about $180,000. Such technology could allow payload capacities of 20, 16, and 12 tons for 300, 600, and 900 mile ranges, respectively. (He concedes that it’s still unclear what benefits in cost might come from Tesla’s increased production at its lithium-ion gigafactory in Nevada.)

The question, then, is whether Musk has enough rabbits left in that hat of his to pull this one off, too.