The visuals are terrific. Imagine the Empire State Building with a windmill on top rather than King Kong. That's how the New York Post depicted Mayor Michael Bloomberg's latest idea. Another illustrator adorned the Brooklyn Bridge with windmills atop its towers.

It's all because Bloomberg proposed that the Big Apple should blossom with windmills to provide at least one-tenth of its power.

What if his idea caught on? Why not mandate that every building taller than a few stories sport a rooftop windmill? We could include the Washington Monument. And every TV and radio antenna. And every hilltop and mountain, including those in national parks.

Don Quixote would be proud. But had Bloomberg done the math, he'd know that even if Manhattan were topped by a solid block of windmills, they wouldn't come close to meeting the city's power consumption.

Wind power has its place as a power source, but it's not a place at the top. It provides less than one-tenth of 1 percent of U.S. electricity because it costs more to produce. The wind may be free, but the equipment is expensive.

The costs are even dearer if you follow Bloomberg's other suggestion, namely floating windmills in the middle of the ocean.

How many windmills does it take to meet the power needs of a typical city, much less New York City?

At www.scitizen.com, Kurt Cobb worked the numbers. Generously, he presumed the windmills would use 5-megawatt turbines - generating three times the output of a typical 1.5-megawatt turbine. He compared that with a 500-megawatt fossil-fuel (coal) power plant needed to power a city of 300,000 people. A typical power plant, he noted, would cover 300 acres, but use only 30 of those for the actual facility.

Cobb calculated it would take 233 5-megawatt wind turbines to equal the coal plant's output, since the wind doesn't blow constantly. Each would need to be spaced 2,065 feet away from the others (five times the diameter of their 413-foot rotors). Adding the rotor diameters to the spacing requirement equates to a 110-mile long line of windmills, half a mile in width.

It comes to 55 square miles. That's to provide electricity for a town of 300,000 people.

New York City has 8.1 million residents. Manhattan Island totals 23 square miles. So, based on Cobb's calculations, it would take six and a half Manhattan Islands, each covered totally with windmills, to power one-tenth of New York City. And if standard 1.5-megawatt wind turbines were used, they would take three times more space.

Mayor Bloomberg's vision is flawed. But it's typical of the pie-in-the-sky energy "solutions" suggested by those who would rather "go green" than "get real."(Bloomberg looks positively reasonable compared to the Australian engineer who proposed a giant helicopter carrying wind rotors 15,000 feet into the sky, and sending back electricity through a tether wire super-sized extension cord!)

Going back to Kurt Cobb's calculations, if we wanted to meet the electric needs of 300 million Americans rather than only 300,000, we'd need a half-mile swath of windmills, each of them hundreds of feet high, 110,000 miles long, crisscrossing the continent 40 times between New York City and Los Angeles.

That's a lot of land to condemn. The cost would be in the hundreds of billions of dollars, since each large windmill costs millions.

National Wind Watch calculates that wind power consumes an average of 50 acres per megawatt when you include the need to remove trees in the vicinity. On that basis, powering America with wind means we'd have to cover Nebraska with wall-to-wall windmills, leaving no room to grow corn (and thus threatening ethanol).

But even if it were practical (and affordable) to convert to wind power on such a massive scale, it generates other consequences, both aesthetic and scientific.

New York State's largest windmill farm to date, the $400 million Maple Ridge project, features 195 medium-size (400-foot high) windmills, part of a windmill surge in upstate New York sparked by state and federal incentives. That project has generated great controversy even in its rural setting. According to area researcher Dr. Nina Pierpont, it has also created "wind turbine syndrome," a variety of ills such as inner ear problems, headaches, difficulty sleeping, ringing in the ears, mood disorders, irritability, panic attacks and child misbehavior, all attributed to the low-frequency rumblings of the windmills.

There are practical problems, too. If the structures are put totally in the boondocks, massive new transmission lines must be built to carry the power to where the people are.

But such large facilities cannot be totally isolated, so NIMBY takes over quickly. Citizens say "Not in My Backyard" to the idea of an ever-present giant whooshing sound. And they oppose the sight as well as the sound.

Yachting liberal Sen. Ted Kennedy, D-Mass., famously took the wind out of the sails to a proposal for offshore windmills in his beloved Nantucket Sound. Walter Cronkite made TV commercials opposing the project, saying, "These massive wind turbines could disrupt the natural habitat for wildlife in the Sound and endanger boats."

Just imagine if the Titanic had been forced to contend with windmills and not just icebergs!

The Nantucket Sound windmills - six and a half miles out - would be visible from shore. The height of such windmills makes them visible from 10 miles away - hence the Bloomberg-ish proposals to site them beyond the horizon, 50 or so miles out. But putting them there - in international waters hundreds of feet deep - raises the costs enormously. Some engineers are designing floating windmills, top-heavy behemoths anchored to the seabed by enormous tethers - with the potential of snapping in rough weather. And they'd still require the super-size extension cords to send their power back to the distant shores.

All this to generate power that is more expensive than our current power plants, even after major taxpayer subsidies are factored in.

Whether onshore or offshore, the gigantic areas involved to harness wind power will bring their own unintended consequences. The director of the Center for Global Change Science at M.I.T, Ron Prinn, warns that erecting enough windmills to replace fossil fuel power has adverse environmental impact. Prinn says those would "take significant momentum out of the atmosphere, so there'd be less penetration of wind to the ground surface." Translation: You change the wind, and you change the weather.

Al Gore, why aren't you speaking up about this?

The environmental community, which for long years could freely propose nice-sounding but impractical solutions, is coming to grips with the reality that wind energy requires an enormous footprint (as does solar energy), far beyond that required by energy-dense and compact fossil and nuclear fuels.

Yet rationality is too much to expect. As a contender for governor of Delaware, Democrat John Carney told a debate audience, "I want to be sure there are enough windmills offshore that there isn't room for drilling."

Let's just hope his windmills don't get in the way of the view, or of Ted Kennedy's sailboat.