Google has agreed to buy up to 407MW of wind energy to power its data center in Iowa, the company announced Tuesday, on Earth Day.

This is its seventh and largest renewable energy purchase agreement, the company said in a blog post. The company sells renewable energy it buys directly from producers but strips the renewable energy credits from it, applying them to regular utility grid energy that actually powers its facilities.

The supplier in the latest contract is MidAmerican Energy. Google said it has been in talks with the renewable energy company ever since it started construction of the Iowa data center in 2007.

The new agreement will cover the internet company's current requirements in the region as well as allow for future expansion.

Google has also invested in wind energy in Oklahoma as well as in Finland, Sweden. Together with the latest agreement in Iowa, the company has now contracted for more than 1,000MW (1GW) of renewable power generation capacity.

Long-term power purchase agreements, such as the new one, often enable the developer to fund construction of the wind farm. Google's rivals Facebook and Microsoft have recently made similar deals with renewable energy suppliers to get renewable energy for their data centers.

Greenpeace gave Google relatively high marks on the latest version of its report card that scores major data center operators' level of commitment to powering their operations with renewable energy.

The environmental activist organization recognized the company for deployment and advocacy for renewable energy, energy efficiency, renewable energy commitment and siting policy and energy transparency.