Facebook will finance construction of six large solar projects to offset power use at its Prineville campus, which the company says will generate enough clean electricity to run all five data centers at the site.

The solar projects - two just south of downtown Prineville and four on PacifiCorp's electrical grid in Utah - will generate 437 megawatts of power. That's an enormous amount of electricity. Facebook says it is equivalent to all the energy its power-hungry computers use in Prineville.

"Cost-effective, renewable energy is one of our priorities at all of our sites," said Peter Freed, Facebook's energy strategy manager.

Facebook has three full-size data centers in Prineville and is building two more. The company has spent more than $1 billion on the projects. It declined to say Wednesday how much it will cost to build the solar projects or how the cost of the clean energy will compare to what it pays now.

Construction is due to begin next year, with all six solar projects generating power by the end of 2020. Facebook won't build the solar projects itself; PacifiCorp will contract with other energy companies that will build, own and operate the sites.

Greenpeace, which began campaigning for Facebook to use renewable energy in Prineville even before its first data center opened, hailed Wednesday's news.

"We think this is a sign of Facebook's strong commitment to address its growing energy demand with renewable energy," Gary Cook, a Greenpeace information technology analyst, wrote in an email Wednesday.

However, he cautioned that PacifiCorp is still reliant on coal-generated electricity across its power grid and said that power will help run Facebook's operations at times its solar projects aren't operating.

The social networking company should address that, Cook said, by making other clean energy purchases and finding ways to store power in pursuit of energy sources that are completely renewable.

Data centers use huge amounts of energy to run and cool the computers inside, which host social networking photos, email and other online data. The solar power for Facebook's Prineville campus is roughly equivalent to the energy use of 100,000 Northwest homes.

Prineville, with just 4,100 households, was home to Facebook's very first data center in 2011.

The company has been continually expanding and is now building its fourth and fifth massive facilities on the bluff above town, drawn back again and again by valuable tax breaks that have saved the company more than $73 million to date.

Facebook, like most large tech companies, accepts the scientific consensus that human activity is responsible for climate change. And like other tech companies, Facebook has been shifting its power load to renewable energy. Apple, which also operates data centers in Prineville, is offsetting its electricity use there with a mix of solar, wind and hydropower.

Facebook already uses renewable energy in data centers elsewhere. Progress had been slower in Oregon because of state regulations around the direct purchase of renewable power by large energy users, according to Scott Bolton, PacifiCorp's vice president of external affairs.

Early last year, Bolton said, PacifiCorp received approval from state regulators for a new tariff structure that enables Facebook and other large power users to buy energy associated with specific, renewable projects.

By paying the cost of the new solar projects, Bolton said Facebook will bring down the cost of renewable energy for other power users.

"I think they're blazing the path to be able to tailor some of these solutions," he said.

This article has been updated with comment from Greenpeace.

-- Mike Rogoway | twitter: @rogoway | 503-294-7699