Environmental groups are accusing Walmart of falling short on a number of its well-publicized sustainability goals. The world’s largest retailer still relies heavily on carbon-intensive coal power in U.S. stores and facilities, despite a longstanding promise to invest in renewable energy, critics said in a report released Thursday.

Only 3 percent of Walmart’s electricity use comes from renewables like solar and wind power, while about 40 percent comes from coal -- an amount equal to about 4.2 million tons each year, according to a new study by the Institute for Local Self-Reliance, a Minneapolis-based advocacy group. Walmart says it aims to use 100 percent renewable energy throughout its global operations.

“Walmart spends a lot of time these days touting its solar projects. The real story of its energy use is buried in a mountain of coal,” Stacy Mitchell, a co-author of the report and senior researcher at the institute, said on a press call.

The study is part of a broader effort by environmentalists to hold corporations accountable on sustainability, a loose term that includes anything from installing rooftop solar panels to boosting in-office recycling or switching to energy-efficient lighting. Green groups target firms that lack environmental commitments and blast companies that do have targets for failing to make progress.

Greenpeace, for instance, released an interactive study in April, called “Clicking Clean,” that scrutinized a dozen technology companies on their climate and energy performance. CDP, formerly the Carbon Disclosure Project, produces annual reports that track how some of the world’s largest companies are addressing the challenge of climate change.

“When you’re a big, branded company like Walmart, you’re a much more visible and desirable target for environmental groups who want to push the agenda,” Andrew Hoffman, a sustainable enterprise professor and expert at the University of Michigan’s Ross School of Business, said by phone. “If you decide to take an aggressive, progressive position on sustainability, that actually increases your chances of being a target, because the more you do, the more you’ll get pushed to do even more.”

Amazon.com Inc. was singled out in the Greenpeace report as being one of the world’s dirtiest technology companies. The Seattle-based retailer later announced it would use 100 percent renewable energy to power its cloud computing division, though it didn’t specify how or by when. A spokesperson for Amazon declined to comment Thursday but provided a link to Amazon Web Services’ sustainability page.

Tara Greco, Walmart's director of sustainability communications, said the retailer is moving closer to its "ambitious" sustainability goals. "We work state-by-state in the U.S., and region-by-region around the world, to make renewable energy work financially and sustainably," she said in an emailed statement. She noted that the Solar Energy Industries Association, a trade group, recognized Walmart as the business with the most installed solar capacity in the U.S., and she added that Walmart is considered the largest on-site renewable energy user in America.

On its website, the company says that renewable energy and energy efficiency “have long been part of Walmart’s strategy to operate at an everyday low cost, while also allowing Walmart to be a good steward of the environment.” Walmart says it has invested in over 335 renewable energy projects globally, and that 24.2 percent of its global electricity needs are met with wind, solar and other clean sources. The company also detailed its strategy for reaching its 100-percent renewables goal in a 90-plus page paper.

On Thursday, SolarCity Corp., the solar developer chaired by Elon Musk, said it had agreed to install solar projects at Walmart facilities in up to 36 states over the next four years. The California company has built more than 200 solar projects on Walmart buildings since 2010, according to a press release.

Even so, Walmart remains one of the largest U.S. consumers of coal-fired electricity, Mitchell said. Her report found that most of Walmart’s domestic renewables projects are in areas where renewables are already abundant and relatively cheap, such as California. In the states where wind or solar would have the biggest difference on the grid, like Missouri, Illinois, Ohio and Texas, the retailer has virtually no clean power projects in place.

“In the places where it matters most, we don’t see this company making any investment in a clean energy future,” Mitchell said.

Major environmental leaders from Sierra Club, 350.org and Green for All joined Mitchell on the Thursday press call. They accused Walmart of “greenwashing,” or making claims about sustainability for branding’s sake without actually making meaningful changes to their energy strategy. They also chided the company for lagging behind its competition in the big-box retail industry.

Swedish furniture giant IKEA says it has plans for $1.9 billion in wind and solar power by the end of 2015, and has already installed solar panels at 90 percent of its stores. On Tuesday, it bought its second U.S. wind farm, a 165-megawatt facility in southern Texas. Kohl’s Corporation has invested in enough renewable energy to equal 105 percent of the electricity it uses at all of its department stores, according to the U.S. Environmental Protection Agency’s Green Power Partnership.

“If Walmart plans to do nothing more than they’re doing now, they should at the very least cease production of press releases and annual reports with pictures of endangered species and solar panels,” Bill McKibben, president and co-founder of 350.org, told reporters. “At this point it’s nothing short of dishonest.”

Hoffman, the sustainable enterprise professor, said that scathing announcements like these aren't necessarily bad for the Walmart brand.

“It can be healthy for the company, in that they’re going to be constantly challenged,” he said. "There’s never going to be a point where [companies] will stop and say, ‘OK, we’re sustainable.’ And that’s not because it’s an insatiable desire” on the part of environmentalists. “The notion that we’re going to reach an end point doesn’t exist.”

Update 11/20/14 6:33 pm EST: The story has been updated to include comments from Walmart spokeswoman Tara Greco.