In January, the U.S. Minerals Management Service issued its final environmental impact statement for a 420-megawatt wind farm off Cape Cod in Nantucket Sound known as Cape Wind.

It would have 130 wind turbines.

Federal approval could come early this year, after a seven-year fight with opponents, including environmentalist Robert F. Kennedy, Jr., who might see the towers on the horizon five miles offshore from their vacation homes.

Will we have the same battles here, or will Marin accept the installation of renewable energy producing facilities "in our backyard"?

Enthusiastic support for the abstract concept of renewable energy sources now meets the reality of what's on the ground.

These alternative plants, whether utilizing wind, solar or other "instant" energy sources, take up massive land area. The reason is that energy from the sun, whether utilized indirectly in the form of wind or directly from the sun's rays, is in its "raw" form. There's a limit to what's available and it must be used now. This is unlike fossil fuels that have stored the energy from millennia of sunny days underground, converted into a highly concentrated form.

Through the efforts of Marin Clean Energy, the cities and the county are forming the Marin Energy Authority to offer consumers a "green energy" alternative to PG&E.

Among the goals of the MEA is to supply 30 percent of Marin's 240 megawatt energy use from wind and 25 percent from solar.

Considering just the wind component, there are obviously many variables that affect the energy produced, from the speed and consistency of the wind to the size and type of turbines. These variables directly affect the amount of land needed for Marin's wind farm.

The U. S. Department of Energy, through its National Renewable Energy Laboratory and other agencies, has published a wealth of information to help answer this question.

When determining turbine density, it is generally assumed that rated production will be about five megawatts per square kilometer. This means that Marin's 30 percent supply goal of 60 megawatts would take 2,970 acres when working at 100-percent capacity.

This is of course not possible. All wind farms have a capacity factor that represents the amount of energy actually produced over a time period compared to the amount produced at full capacity. This factor ranges from 25 percent to 40 percent. Using a 35 percent factor, about 8,500 acres (over 13 square miles) would be needed to meet just 30 percent of Marin's already miniscule energy needs.

Is it fair for Marin residents to ask other communities to accept this plant in their neighborhood (and the transmission lines)?

The responsible answer is no, particularly since Marin has good wind resources just off its coast.

According to the California Energy Commission, the mean wind speed off the County's north coast is about 16 mph - well above the threshold level needed. We also are fortunate that the ocean floor in this area, a mile or so off the coast, is within the depth of 30 meters (16 fathoms) or less that is suitable for free-standing tower foundations.

Offshore installations like Cape Wind, and possibly here, lend themselves to the use of fewer but larger turbines. For example, General Electric, a leading supplier, offers a 3.6-megawatt rated capacity turbine. The rotor has three blades sweeping a diameter of 341 feet. Tower height is site dependent, but in an offshore setting would probably be about 300 feet tall. It is impossible to estimate how many of these turbines would be needed here without extensive studies, but it is likely to be less than 50. More smaller, lower capacity turbines might also be the answer once these studies are done.

Important decisions will have to be made soon.

Will Marin put its land where its commitment is?

We'll see.

Peter Van Meter is an engineer and a former Sausalito City Councilmember who has been active in planning and land use issues for over 30 years.