At the grand opening of Tesla's enormous Gigafactory in July, CEO Elon Musk said he wants to build Gigafactories on several continents. He told BBC he wanted a factory "in Europe, in India, in China ... ultimately, wherever there is a huge amount of demand for the end product."

Well, it looks like Musk's factory-building plans are well underway.

The company said in its fourth-quarter investor letter on Wednesday that it is considering building up to five Gigafactories.

The letter states:

"Installation of Model 3 manufacturing equipment is underway in Fremont and at Gigafactory 1, where in January, we began production of battery cells for energy storage products, which have the same form-factor as the cells that will be used in Model 3. Later this year, we expect to finalize locations for Gigafactories 3, 4 and possibly 5 (Gigafactory 2 is the Tesla solar plant in New York)."

Tesla officially flicked on Gigafactory 1's switch in January. The factory produces lithium-ion battery cells for Tesla's suite of battery storage products, the Powerwall 2 and Powerpack 2, as well as the company's mass-market electric car, the Model 3.

Gigafactory 1 is currently being built in phases so that the company and its partners can manufacture products while the building continues to expand. Construction is expected for completion by 2018, at which point the plant could claim the title of world's largest building by footprint.

The facility will also be astoundingly clean and energy efficient, as it will be powered 100 percent by renewables such as solar, wind, geothermal and will feature energy-storage technology.

The company also plans for the building to achieve net zero energy. Tesla co-founder and chief technical officer JB Straubel once explained why Tesla wanted Gigafactory operations to be completely carbon neutral:

"The Gigafactory is maybe the best example we can talk about with this. You know, from the get-go, from the first concept of this factory, we wanted to make it a net-zero facility. So, you know, the most visible thing we are doing is covering the entire site with solar power. The whole roof of the Gigafactory was designed from the beginning with solar in mind. We kept all of the mechanical equipment off the roof. We didn't put extra, sorta, penetrations through the roof that we didn't need to and it's a very, very clean surface that we can completely cover in solar. But that's not enough solar, though. So we have also gone to the surrounding hillsides that we can't use for other functions and we're adding solar to those."

According to Straubel, the Gigafactory isn't even hooked up to any natural gas pipelines:

"The other interesting thing is we wanted to manage the emissions from the Gigafactory. Solar power can do some of that, but we took kind of a radical move in the beginning and said we are not going to burn any fossil fuels in the factory. You know, zero emissions. We are going to build a zero-emissions factory—just like the car. So, instead of kind of fighting this battle in hindsight, we just said we are not even going to have a natural gas pipeline coming to the factory, so we didn't even build it. And it kind of forced the issue. When you don't have natural gas, you know, none of the engineers can say, 'Oh, but it will be more efficient, let me use just a little bit.' Sorry, we don't even have it."

In December, Tesla and Panasonic launched operations at its Buffalo, New York plant, now dubbed Gigafactory 2. The factory manufactures high-efficiency photovoltaic cells and modules for solar panels and solar glass tiles for Tesla's highly anticipated solar roof.

Tesla's factories are all part of the company's mission to accelerate the world's transition to sustainable energy.

In last year's climate change documentary Before The Flood, Musk takes Leonardo DiCaprio on a tour of Tesla's massive Gigafactory in Nevada. During their chat, the Tesla CEO tells the actor and famed environmentalist that it would only take 100 Gigafactories to transition "the whole world" to sustainable energy.

With at least five Gigafactories in the books, looks like Musk's plans are slowly becoming reality. For what it's worth, even DiCaprio said building one-hundred Gigafactories "sounds manageable."