In a recent article about the average cost of EV batteries, we quoted one source as saying Audi has an acquisition price of $114/kWh. That’s a significant bit of information where Tesla Motors is concerned because Elon Musk categorically said last month that the Model 3 battery has the best cost. Period.

Logic tells us that Tesla’s cost is even lower than that of what Audi claims it’s going to put into the e-tron quattro, but the Model 3 battery still hasn’t hit the magical $100/kWh price point.

Why is that number important? Because that’s when the Model 3 starts becoming profitable. It’s either that or being able to produce 10,000 cars a week, which is double what they expect to reach by the end of June 2018.

It is also significant in that a price point of $100/kWh will make the Tesla Semi a viable product.

Class 8 trucks are in that category because of their Gross Vehicle Weight (GVW) of 80,000 pounds, a figure freely bandied about by Musk at the truck’s launch last year. But nobody outside Tesla seems to know the Tare Weight (GVTW), which is actually the more important number because it tells a shipper how much of a load the truck can carry.

An average Class 8 cab with a light trailer fully loaded with gas weighs in at about 32,000 pounds, which means it can theoretically carry up to 48,000 pounds worth of freight. Freight shippers typically load their trucks to the maximum allowable weight in order to minimize cost, which is why nobody is going to buy a Tesla Semi unless the tare weight is at or under 32,000 pounds.

And that’s why battery cost – and, more importantly, battery density – is so critical, because the entire fate of the Tesla Semi line rests whether or not the trucks can carry comparable loads at prices at or lower than traditional diesel trucks.

We know that Tesla has achieved the best battery density and lowest battery cost in the industry, but what are those numbers? Without that, it’s going to be a hard sell for Tesla Semis and a tough proposition for the company to make a buck on the Model 3.