The New York Power Authority took its first major step Tuesday to advance its efforts to encourage the development of a big wind farm off the shores of Lake Erie or Lake Ontario.

The authority formally began the process of seeking proposals from developers interested in building the wind energy project, which could generate between 120 megawatts to 500 megawatts of electricity.

The project, depending on its ultimate size, would be six to 25 times the size of the Steel Winds wind energy project on the site of the former Bethlehem Steel plant in Lackawanna. And while the Steel Winds project has eight windmills, the offshore wind farm could have anywhere from 40 to more than 150 turbines spinning from towers located in the waters two to six miles from the shoreline.

Even under ideal circumstances, the wind farm, estimated to cost upwards of $1 billion for a larger-scale project, likely would not begin operation until 2015. But Richard M. Kessel, the Power Authority’s president and chief executive officer, said it still could be one of the first — if not the first — offshore wind power projects in the United States.

“Maybe this could be the first. How great would that be?” Kessel said.

The Power Authority first announced its interest in supporting a major offshore wind farm along the Great Lakes in April. The agency since then has conducted preliminary studies to identify potential sites along much of New York’s Lake Erie coastline, as well as along portions of Lake Ontario, primarily north of Youngstown, Rochester and Sodus in Wayne County and on most of the lake’s eastern edge.

“I would love to see multiple sites,” Kessel said. “I think the idea of having turbines on both lakes would be terrific.”

Kessel said it is possible that multiple sites could be selected for the wind farm, which likely could be built over a two-year period.

“I think it’s time to move,” Kessel said. “Now it’s time for the competitive process to unfold.”

Power Authority officials touted the offshore wind farm as a potential economic boom for upstate New York, creating thousands of jobs through the project’s construction, as well as the production of components used in the turbines and towers.

Developers who commit to using local labor and materials in the project will receive preference under the authority’s evaluation process, as will proposals that would make some of the equipment in upstate New York, Kessel said.

Roughly 14 to 15 potential developers have expressed early interest in the wind farm project, Kessel said. The authority set a June 1, 2010, deadline for developers to submit proposals, with a goal of selecting a developer by the end of next year.

A key element would be for the developer to reach an agreement to sell the electricity generated by the wind farm to the Power Authority for a period of up to 20 years. That agreement is essential because it would give the developer an established revenue stream that will be essential if the project is to obtain financing.

The Power Authority’s participation also is important because the electricity generated by the wind farm is expected to be much more costly than power currently produced by conventional sources in New York. Depending on the project’s costs, that electricity could cost roughly 50 percent more than current rates.

But Kessel said the project, despite its current cost disadvantages, would have other economic benefits, including the production of pollution-free electricity.

The project also would face other sensitive issues, ranging from the impact the giant spinning turbines, standing in 50 to 150 feet of water, would have on bird populations. How the towers would affect shipping lanes also is a consideration. The impact the wind farm would have on the aesthetics of the shoreline also would likely be a significant issue.

Terry Yonker, the U. S. cochairman for the Great Lakes Wind Collaborative, said it is important for the Power Authority to make good choices as it proceeds with the project because it will set precedents for other offshore wind farms in the Great Lakes region.

“The potential for wind power in the Great Lakes is among the best anywhere on earth,” he said.