A Tesla semi might come with 500 kWh of battery and a number of DC charge points. It might go 500 km loaded.
500 km is about 310.7 miles. If you presume there is perhaps a 5% buffer for anti-brick protection, the 500 kWh battery pack would have 475 kWh available for use. That works out to about 1,528.9 Wh per mile, and possibly as low as 1448.2 Wh per mile
(with 450 kWh usable, a 10% buffer).
According to
Popular Mechanics:
In 1973, the feds estimated that semis got about 5.6 miles per gallon of diesel; today's estimate is 6.5 mpg, although different trucks get fuel economy in a range from 4 to 8. Going up a steep hill, a truck's mileage might drop to about 2.9 mpg, while going down the same hill will raise it to more than 23 mpg.
A U.S. Gallon of diesel fuel holds 40.7 kWh of energy. So in a diesel semi truck, at 8 MPG, that is 38.8 gallons to cover 310.7 miles (500 km), or an expenditure of 1,580.6 kWh, and that works out to 5,087.5 Wh per mile. Of course, quite a bit of that energy is used up by creating heat, light, vibration, and sound... But if one presumes the diesel drivetrain is as much as 38% efficient
(they aren't, that would put them about the same as a Prius), then the work of travelling the 310.7 mile distance is accomplished using only 1,933.25 Wh per mile, or a total of 600.6 kWh of energy. With that in mind, a battery powered drivetrain rated at only 85% efficient would need maybe a 742 kWh battery pack to ensure covering the range. That would be effectively the energy capacity of 18 gallons of diesel fuel, or around 22 gallons of gasoline. Personally, I'd hope Tesla would aim for a semi truck that was capable of 900 to 1,200 Wh per mile in the majority of situations. Even then, you might need 900 kWh of capacity to cover 600+ miles. So, perhaps they'll start with a nice 1,000 kWh or 1 MWh capacity.
Being a truck, there is nothing to stop a driver carrying, say, a 100 kW generator on the trailer and wiring it in to the battery for on the move charging when needed. Vehicles designed to carry freight can be regarded as a hybrid due to their capacity to carry a portable generator. Electric semi-trailer manufacturers should wire their vehicles so an operator can plug-in a generator for on the more charging. Tesla should do it to.
Industrial level generators are best left in stationary applications. And if they are stationary, you'd might as well use a Supercharger or Battery Swap. Tesla Motors has no intention of giving any validity whatsoever to hybrid applications. Their position is that all hybrids are essentially a stop-gap
'solution' at best, and a delaying tactic at worst. If someone else wants to build the
best possible hybrid semi truck, Tesla will simply aim to prove a fully electric semi truck would be
BETTER.