As of now, the top three most widely used search engines, by a considerable margin, are Google, Yahoo and Microsoft's Bing.

This we all know. But what few people realise is that if you are using these searches, it is growing more and more likely that you are also engaging in what is, in effect, a green pattern of Internet use.

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The reason? Google, Yahoo and Microsoft are part of a growing number of tech and other major companies that are entering into long-term "power purchase" agreements (PPAs) with wind farms to ensure a steady stream of power, at a fixed cost, over a period as long as several decades.

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Most recently, last month Yahoo signed such a deal for wind power in the Great Plains with OwnEnergy, a wind energy developer.

Google - which is already carbon neutral and now trying to power itself with "100 per cent renewable energy" - has the longest history here. It has three PPA deals in the US wind sector (in Iowa, Oklahoma, and Texas), and two more in Sweden.

Microsoft, meanwhile, currently has two PPA deals with wind installments located near its data centres in Texas and Illinois. The agreements provide 285 megawatts of power to help drive both Bing searches and also its other online platforms, according to Brian Janous, the company's director of energy strategy.

What these deals have in common is that they involve purchasing clean energy in close proximity to the power hungry data centres that these companies operate - data centres that in turn drive searches, apps like Gmail and much more. "These are very energy intensive operations that these companies are planning on running for years, and they know they need electricity," says Emily Williams of the American Wind Energy Association.

Clearly, Google is the leader here. Indeed, the company has a very advanced philosophy for its clean energy acquisitions, explaining in detail why it thinks these PPA arrangements are the way to go.

"Because we've purchased 1,000 megawatts of renewable wind energy for our data centres, you might say using Google is like kite-surfing the internet," said a Google spokesperson.

So does this mean that your search is, very literally, powered by clean energy - at least some of the time?

In part due to the physics of energy itself, and in part because of the structure of these deals, that question requires a pretty nuanced answer. Unlike with the installation of, say, rooftop solar, in a PPA arrangement companies are not directly plugging their facilities into a wind farm that they actually own.

Google argues that that idea doesn't even really make sense - the best place for siting a data centre and the best place for siting a vast wind farm are rarely the same.

So it's not that the wind power goes directly to the data centre. Rather, the companies buy their power most directly from a utility, just like the rest of us. However, through their power purchase agreement, they have provided a fixed stream of payments to a wind farm that is putting power onto the same grid that their data centre is sucking power off of.

Moreover, this long-term agreement provides enough financial stability that it allows the wind farm to be built in the first place, or for further expansion of the wind industry. Google calls the concept "additionality," meaning that the company can prove that its actions added more overall wind power to the world.

"What's really important to sustainability managers in these companies is to be able to say, we went so far as to enter into a direct contract because we knew that it was going to lead to a new wind farm being built," says Jacob Susman, CEO of OwnEnergy, which did the Yahoo deal.

In Microsoft's case, explains Janous, the push into wind was driven by the company's own internal imposition of a carbon tax on company operations. This allowed for easier calculation of the financial benefit that clean energy provides. Plus, with the growth of cloud computing, energy costs became paramount.

"I think there was a recognition, to the executive level, that energy costs are becoming an increasingly important part of our business, because energy is the raw material of the cloud," says Janous. Power purchase agreements provide long term energy price stability - and in the future, Janous thinks they'll increasingly be used to acquire solar energy, as well as wind.