This article from John Voelcker of Green Car Reports puts the Tesla factory in an interesting light. Today Tesla uses 1/3 of the world's electric-car batteries. To scale to the 500,000 cars they hope to eventually sell, then need the gigafactory. See the excerpt from John's article below.
Tesla Gigafactory Faces Skepticism From Panasonic, Analysts
Musk announced a handful of high-level details about the proposed plant, which could be the world's largest single factory, in late February.
The need is clear: Even at a production rate of about 25,000 cars a year, Tesla used one-third of the world's electric-car batteries.
To scale up to 100,000 Model S sedans and Model X crossovers by 2016, he needs to quadruple that supply--which Panasonic and Tesla together expect to happen.
But if the company starts production of its next model, a lower-cost sedan often dubbed the "Model E," it will need multiples of its current battery supply.
And to hit the price of $35,000 or less for the new car, which is expected to offer at least 200 miles of rated range, Tesla will need every bit of the 30-percent cost reduction it says it can achieve with the gigafactory.