Musk was quoted as saying “We actually did the calculations to figure out what it would take to transition the whole world to sustainable energy. You’d need 100 Gigafactories.”

I wonder what they actually calculated. You can measure/calculate the output of the factory in terms KWH of storage capacity produced daily. But what metric characterizes "the whole world" that you could then compare that number to in order to decide that 100 factories are enough?

That's right. Musk himself didn't say "power", though.