Major US corporations such as Wal-Mart Stores and General Motors have become some of America's biggest buyers of renewable energy, driving growth in an industry seen as key to helping the US cut carbon emissions.

Last year nearly 40 per cent of US wind contracts were signed by corporate power users, along with university and military customers.

That's up from just five per cent in 2013, according to the American Wind Energy Association trade group.

These users also accounted for an unprecedented 10 per cent of the market for large scale solar projects in 2016, figures from research firm GTM Research show. Just two years earlier there were none.

The big reason is lower energy bills.

Costs for solar and wind are plunging thanks to technological advances and increased global production of panels and turbines.

Coupled with tax breaks and other incentives, big energy users such as GM are finding renewables to be competitive with, and often cheaper than, conventional sources of electricity.

The automaker has struck deals with two Texas wind farms that will soon provide enough energy to power over a dozen GM facilities, including the US sport utility vehicle assembly plant in Arlington, Texas.

The company is already saving $US5 million a year worldwide, according to Rob Threlkeld, GM's global manager of renewable energy, and has committed to obtaining 100 per cent of its power from clean sources by 2050.

"It's been primarily all driven off economics," Threlkeld said.

"Wind and solar costs are coming down so fast that it made it feasible."

Growing corporate demand for green energy comes as US President Donald Trump is championing fossil fuels and targeting environmental regulations as job killers.

Over the past four years, corporations have contracted for about 7 gigawatts of renewable energy - enough to power more than 1 million homes.

That number is expected to rise to 60 GW by 2025, according to the Edison Foundation Institute for Electric Innovation, a utility-backed non-profit based in Washington DC.

Early corporate adopters included Alphabet and Amazon.com, leading-edge companies with progressive company cultures, deep pockets and major power needs.

Now mainstream industries are stepping in as costs have plummeted.

Wind-power costs have dropped 66 per cent since 2009, according to the American Wind Energy Association, while the cost to install solar has declined 70 per cent since 2010, according to the Solar Energy Industries Association trade group.

Demand from big corporations has benefited a host of wind and solar developers including Pattern Energy, First Solar and NextEra Energy.

BNB Renewable Energy Holdings, a privately held New York-based developer, said corporations now make up about half its business.

"There is a convergence right now where price is low and their sustainability commitments are high," said Jos Nicholas, a managing partner with BNB.

The developers or owners of the projects, meanwhile, get the stability of long-term contracts plus those federal tax breaks.

The solar credit is worth up to 30 per cent of a project's value.

For wind, the most popular tax credit is a maximum of 2.4 cents per kilowatt-hour of electricity produced for a decade.

Since 2014, nearly 100 large global companies have committed to transitioning to 100 per cent renewables through a partnership with The Climate Group, a nonprofit that's working to reduce greenhouse gas emissions.

Roughly two corporations a month are joining that effort, according to Amy Davidsen, the organisation's executive director for North America.

In addition to GM, US companies that have made the commitment include Johnson & Johnson, Procter & Gamble and Nike.

Still, many big firms remain on the sidelines because they lack an overall corporate sustainability mandate, view renewables as having unattractive returns or because the contracts are too long, according to a 2016 PricewaterhouseCoopers survey.

AAP

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