The Tesla big rig is going to need a big battery. Even it doesn't offer ranges on the order of 1,000 miles per charge (what some diesel rigs can deliver today), it's still going to require the torque and range to serve up something like 200-300 miles per charge to make sense.
Luckily, the conventional shape and size of the semi lends itself to a huge battery pack. Tesla's biggest pack for a passenger vehicle is a 100 kilowatt-hour unit. It fills the floor of a Model S or Model X.
But Tesla can do larger batteries, for its utility grade Powerpack applications. Powerpack 2 is a 200 kWh lithium-ion unit. And the engineering of a big rig, if it's all electric, is fairly modular. You need a platform, perhaps two electric motors to power all four wheels, and cab — leaving the motor-free leftover space available for batteries.
The torque should be insane — and torque is what big rigs require, as they're towing massive amounts of weight. Mack's MP8 engine, for example, is a Class-8 diesel powerplant that makes upwards of 505 horsepower, but more importantly, over 1,800 pound-feet of torque.
A Tesla Model S P100D can do almost 800 pound-feet of torque. This is speculative, but a pair of 100-kWh batteries could double that torque output.
Charging a battery that large could take a consumer a couple of hours at a Tesla Supercharger modified for big rigs, but trucking is more flexible when it comes to this kind of scheduling that folks taking long trips in their personal cars, who just want to get where they're going. The logistics industry is an ideal realm for computing power to schedule and optimize freight routes and pickup and delivery times.
Freight companies are also in a better position to absorb substantial battery replacement costs. So ironically, Tesla's technologies could make more sense in a big than in a personal car.