Google is investing a further US$600m into its Caldwell County data center in Lenoir, North Carolina, following a pledge from local energy provider Duke Energy to develop a program for large companies to purchase renewable energy.

The investment is on top of the initial US$600m spent on the operational facility, which broke ground six years ago.

The data center is used for housing computer system and providing services including Gmail, Google + and Youtube.

Google said it originally chose its site at Lenior because of the region’s energy infrastructure, developable land and available workforce.

It has already worked with Duke Energy and Duke University on a project that has seen hog waste at Yadkin County farm become renewable energy.

In a whitepaper released Friday, it said the introduction of renewable energy tariffs will help it to increase its use of renewables.

Google said such a program should also provide customers with a way of choosing the renewable energy source they wish to procure and that it should ensure transparent pricing.

“We believe that a renewable energy tariff will provide a powerful tool for accelerating growth of renewable energy,” Google said in the whitepaper.

“It offers a solid and workable framework for both utilities and their large customers seeking to procure renewables.”

Google said it is working with Duke Energy on a voluntary model that will go for regulatory filing with the North Carolina Utilities Commission within the next three months.

Google Global Infrastructure team director Gary Demasi said the options will be developed not only for Google but other major electricity users.

“As more of the world moves online, demand for Google’s services continues to grow – and we want our renewable energy options to grow with it,” Demasi said.

Google has carried renewable energy generation on site in the past – it has a 1.7MW solar array at its Mountain View headquarters for corporate operations. But it said it can often be better placed offsite, and closer to the renewable energy resource, for facilities with larger electricity loads like data centers.

While it said many companies procure renewable energy using RECs (Renewable Energy Certificates) these do not provide an assurance on price.

“This solution has worked well for Google because it adheres to our strict standards of additionality,” Google said.

Google has signed two long-term power purchase agreements using this model, for more than 100MW each for its Iowa and Oklahoma data centers,  but it said the process of managing power procurements on the wholesale energy markets can still be a complex process.

So far, Google said it has committed more than US$1bn to renewable energy project investments and signed agreements to procure more than 260MW of wind power near its data centers and 1.7Mw of solar power.