A Greenpeace report into the green credentials of tech firms has singled out Amazon as having the "dirtiest cloud" services.
Apple, Facebook and Google were praised for "significant improvements" in energy transparency and attempts to move to 100% renewable energy.
But Amazon Web Services (AWS), which powers many net firms, only uses 15% clean energy, according to the report.
The retail giant hit back, branding the report "inaccurate".
The green activist group warned that more needed to be done to make the internet greener, particularly in countries experiencing huge net growth such as China.
The electricity demand of internet and cloud services has grown exponentially, according to the report, Clicking Clean: How Companies are Creating the Green Internet.
And it is expected to increase by 60% or more by 2020 as reliance on the internet increases.
"Apple, Facebook and Google are powering our online lives with clean energy and building a greener offline world for everyone in the process," said Gary Cook, Greenpeace's senior IT analyst.
It represents a turnaround for Apple which two years ago was criticised in a 2012 Greenpeace report, How Green is Your Cloud.
The firm went from a 35% use of renewable energy in 2010 to 75% by the end of 2012.
Amazon though has remained on Greenpeace's blacklist.
Amazon Web Services, which hosts a large part of the internet including for companies such as Netflix, Spotify, Tumblr, AirBnB and Pinterest, currently only sources 15% of its electricity demand with clean energy, according to the report.
Coal powers 28% of the company's cloud, nuclear 27% and gas 25%.
"By continuing to buy dirty energy, Amazon Web Services not only can't seem to keep up with Apple, but is dragging much of the internet down with it," Mr Cook said.
Greenpeace also criticised Amazon and Twitter for their refusal to reveal any details about their energy footprint to their customers or the public.
In response, Amazon told the BBC: "Greenpeace's report misses the mark by using false assumptions on AWS operations and inaccurate data on AWS energy consumption."
It was not able to elaborate on what the inaccuracies were but said that two of its data centres used 100% carbon-free power.
"Running IT infrastructure on the AWS Cloud is inherently more energy efficient than traditional computing that depends on small, inefficient, and over-provisioned data centres," it said.
Of the 19 leading internet companies evaluated by Greenpeace, only five have committed to a goal of powering their operations with 100% renewable energy.
Apple was the first firm to achieve its 100% renewable energy goal. It operates the largest privately owned solar installation in the US at its North Carolina data centre.
Meanwhile Facebook is powering its utility in Iowa with wind energy, which in turn prompted energy firm MidAmerican to invest $1.9bn (£1.1bn) in wind power energy.
Google has also used wind energy to provide electricity for services such as Gmail and YouTube.