When Biogen Idec Inc. opens a sixth building at its Kendall Square campus this winter, there will be something unusual in the basement: a power plant big enough to light up 4,000 homes.

Two years ago, the electric utility NStar infuriated environmentalists by thwarting a similar off-grid power plant project proposed by Boston University, demanding enormous fees for BU to keep a standby NStar connection.

Now, Biogen Idec is the first company to take advantage of new state rules adopted since then, after intense negotiations with NStar and other utilities, to promote so-called distributed generation projects.

"Green power" advocates hope the prominent biotechnology company will set a trend and assure other businesses of the cost and environmental benefits of distributed generation. Biogen Idec's unit will convert gas into steam for all six buildings and generate up to 5,500 kilowatts of electricity, while also powering air conditioning systems in the summer.

Because the system operates at 80 percent efficiency, compared to 40 to 50 percent for conventional generating stations, Biogen Idec expects to save up to $4 million annually on utility bills. Also, the more efficient generators will keep 25,000 tons of carbon dioxide out of the atmosphere every year, said Brian Casey, chief executive of SourceOne Inc. The Boston energy management consulting firm is overseeing the project for Biogen Idec.

But some environmentalists fear other businesses and institutions hoping to follow Biogen Idec will face an uphill fight with NStar, which could face increasing pressure on its $3 billion annual revenues and $188 million annual profits if more and more big customers defect to distributed generation.

"I have no reason to believe that NStar has seen the light on the value and importance of distributed generation, despite the clear and obvious benefits it offers for everyone," said Seth Kaplan, a senior attorney with the Conservation Law Foundation who helped negotiate the new rates and policies for distributed generation. "NStar has not shown as much receptivity as they should be showing to the role DG can play."

NStar spokeswoman Caroline Allen said that "our support for distributed generation speaks for itself." Across its system, NStar has approved more than 186 off-grid power projects, including 132 solar installations. "This is a commitment that we've made and that we're excited about," Allen said.

But within NStar's service territory, covering 1 million businesses and homes in Greater Boston, Biogen Idec's project is the only businessowned distributed generation project underway. Wind turbines are in the works at Cape Cod Community College in Barnstable and the Massachusetts Maritime Academy in Bourne, along with a New Bedford project generating power from landfill gas.

Under the new state rules, Biogen projects will only have to pay NStar $400,000 to $500,000 for a backup connection to the utility grid when its plant shuts down or requires maintenance, plus the cost of electricity it then uses.

NStar had opposed the BU project, which would have included a tennis court-size fuel cell and research facility, because it said it feared that ratepayers would be stuck with too much of the cost of maintaining the local power grid if BU switched to taking NStar power only during emergencies.

"The new standby rate addressed those concerns for us, Allen said, referring to action by the state Department of Telecommunications and Energy.

Paul W. Gromer, Executive Director of the Solar Energy Business Association of New England, said that "NStar and other utilities have become increasingly comfortable with DG since a separate negotiation last year established firm standards for how small off-grid power projects can safely connect to the utility and sell back surplus power."

Kate Warner, an architect and solar energy proponent who founded the Vineyard Energy Project on Martha's Vineyard, said that eight years ago homeowners installing solar panels often had to meet three or four times with utility engineers for approval.

"They definitely have made progress by going to a standardized application process, Warner said, although advocates still chafe at state rules restricting the size of off-grid projects that schools and businesses can connect to NStar. For Biogen Idec, saving the planet has only been an afterthought. Initially, said company Facilities Director Ed Dondero, Biogen only sought to take over operations of the steam system serving its buildings after the steam utility spun through several owners and Mirant Corp., which owns the power plant that makes the steam, went bankrupt.

"It wasn't a business that we wanted to get into, but the further we got into it, the more sense it made, Dondero said. The locally produced electricity promises to be, compared to NStar, more reliable and less volatile, a crucial consideration for a biotech company relying on high-tech computerized manufacturing systems that can be damaged by erratic currents. Also, unlike an old-fashioned factory where a one-hour outage would only shut down one hour's worth of production, at Biogen Idec a one-hour power outage could destroy hundreds of thousands of dollars worth of biological medical products that are made in 24- to 36-day batches.

To keep the gas-fired generating unit from deafening and rattling researchers and executives in the six upper stories, Biogen Idec's architects have designed the "Bio 6A" building with features like a special steel skeleton for the power gear structurally separated from the main building frame, and a first-floor slab that effectively floats on a layer of noise and vibration-absorbing neoprene.

Executives at the biotech company have had to learn far more about the world of energy production and generator design than they ever wanted, Dondero said, but added: "Now that we're here, we're really excited about it."