It’s not just the range — it’s the weight, too. Oh, and don’t forget about cost. These are some of the potential stumbling blocks facing Tesla’s introduction of an electric semi truck, say Carnegie Mellon University researchers in a peer-reviewed study expected later this month.

Tesla has two trucks up its sleeve. One, an electric big rig, is slated for reveal this September, while an electric pickup should appear within the next two years. So far, it’s looking like the latter vehicle is the viable one.

The study, to be published in the American Chemical Society’s ACS Energy Letters, was previewed by Wired. In it, the researchers call into question just how practical an electric, long-haul 18-wheeler can be.

“The challenge is on par in difficulty level with electric airplanes,” said Venkat Viswanathan, who crafted the study with colleague Shashank Sripad.

There isn’t much known about the looming big rig, except that it will use the same motors as the upcoming Model 3 sedan. Based on this information, and using the current Tesla standard of a battery pack generating 243 watt-hours per kilogram, the researchers examined all of the factors affecting transport trucks: anticipated load, distance traveled, aerodynamic drag, etc.

A typical semi covers between 300 and 600 miles a day. To cover 600 miles without charging, the truck would need a 14-ton battery, the study claims. Boost the range to 900 miles, and the battery would tip the scales at 22 tons. While battery prices are trending downwards, current prices state the packs would carry a price tag of $290,000 to $450,000 alone, minus the cost of the overall vehicle. Compare that to the price of a regular diesel semi — roughly $120,000.

Because federal laws limit a truck’s gross weight to 40 tons, a Tesla big rig configured in such a manner would likely only be able to haul 9 tons of cargo — far less than the average payload of 16 tons. When you add to that the inflated price, you’re left wondering what shipping company would pay for such a vehicle. Yes, electric vehicles are touted as having lower ownership and maintenance costs, but such a truck would have to be on the road for a very long time before making up the difference. As well, there’s the issue of recharging times.

“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.”

Advancements in battery technology will ultimately breed lighter packs with greater range, but the great leap forward in stored energy hasn’t yet occurred. At least, not on a mass-produced basis. The study’s authors anticipate a “beyond lithium-ion battery pack” will one day make an electric semi capable of hauling a full load the desired distance.