Construction is set to begin today on seven towering windmills that together will constitute the largest wind power project in the Eastern U.S. Most wind farms in the nation have been built by utilities, but this $16 million project planned for the rural town of Madison, N.Y., is being built on speculation, without a guaranteed set of customers, by PG&E Generating, which sells power on the open market. The company expects the wind farm’s customers to be companies and institutions like universities that are willing pay an additional 5 or 10 percent on their electric bills in order to say that they are using clean power. Local officials and farmers who have leased land for the project expect it to boost the local economy.