Apple will invest up to US$850 million to build a massive solar farm in Monterey County, California to ensure its future power supplies.

The 2,900 acre California Flats Solar Project will provide enough energy for Apple's future head office in Cupertino, said Apple CEO Tim Cook. There will also be enough leftover power for 60,000 nearby homes.

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Cook described the solar farm as his company's "biggest, boldest and most ambitious project ever". The solar farm will provide all of Apple's energy needs in California, including the Apple Campus 2, other California corporate offices, the data center in Newark, California and the 52 Apple stores in California.

The Apple solar farm will have a generating capacity of 130 megawatts. It will be the largest in the tech industry to provide clean energy to a commercial end user.

Apple currently uses renewable energy to power its data centers. Apple last week announced an investment of $2 billion over 10 years to convert a closed sapphire glass plant in Arizona into a data center powered mostly by solar energy.

"We know in Apple that climate change is real. The time for talk is passed," Cook said. "The time for action is now."

Construction of the solar farm is due to begin the middle of this year and finish by December. The solar panels will be supplied by First Solar based in Tempe, Arizona. This firm manufactures solar panels and builds solar power plants.

"It's one thing to talk about being 100 percent renewably powered, but it's quite another thing to make good on that commitment with the incredible speed and integrity that Apple has shown in the past two years," said environmental group Greenpeace in a statement.