Google‘s data centres and the offices for its 60,000 staff will be powered entirely by renewable energy from next year, in what the company has called a “landmark moment”.

The internet giant is already the world’s biggest corporate buyer of renewable electricity, last year buying 44% of its power from wind and solar farms. Now it will be 100%, and an executive said it would not rule out investing in nuclear power in the future, too.

“We are convinced this is good for business, this is not about greenwashing. This is about locking in prices for us in the long term. Increasingly, renewable energy is the lowest cost option,” said Marc Oman, EU energy lead at Google. “Our founders are convinced climate change is a real, immediate threat, so we have to do our part.”

Technology companies have come under increasing scrutiny over the carbon footprint of their operations, which have grown so fast they now account for about 2% of global greenhouse gas emissions , rivalling the aviation industry.

Oman said it had taken Google five years to reach the 100% target, set in 2012, because of the complexity involved with negotiating power purchase agreements. “It’s complicated, it’s not for everyone: smaller companies will struggle with the documents. We are buying power in a lot of different jurisdictions, so you can’t just copy and paste agreements.”

The company’s biggest demand for energy is its data centres and it admits their overall thirst for power is growing, despite experiments to improve their efficiency through AI .

In 2015, Google bought 5.7 terawatt hours (TWh) of renewable electricity, a little less than the 7.6TWh generated by all of the UK’s solar panels that year . The majority of the power comes from windfarms in the US.