Republican presidential candidate Senator John McCain travelled to Oregon in mid-May to deliver the definitive climate change speech of his campaign. He spoke in Portland, at the U.S. headquarters of Vestas Wind Systems AS, a Danish company that markets wind turbines around the world. He started on a self-deprecating note. "Today is a kind of test run for this company," he said. "They've got wind technicians here, wind studies and all these wind turbines. But there's no wind. So now I know why they asked me to come and give a speech."

It was perhaps his most perceptive statement of the day. Five sentences later, Mr. McCain made perhaps his least perceptive. "Wind," he said, "is a predictable source of energy."

Really? Define predictable. Wind turbines operate occasionally with remarkable efficiency at 100 per cent capacity. More often, they operate with 20 per cent capacity. Once in a while, they operate with subzero capacity - taking electricity from the grid to keep themselves running until they get hit again by a restless wind.

British energy consultant Hugh Sharman, based in Denmark, documented wind power's capacity for subzero performance in a report published by Civil Engineering magazine in 2005. With more wind power per capita than any other country, Denmark (population 5.4 million) is the world's showroom nation for this highly fashionable form of renewable energy.

Liberal Leader Stéphane Dion (who used Earth Day to champion wind power this year) and legendary U.S. oilman T. Boone Pickens (who called this week for massive U.S. investment in wind power) illustrate the widespread popularity of wind power.

Although the United States and Germany generate more wind power in absolute terms, Denmark boasts the world's greatest "wind density" - wind power per capita. With 19 per cent of its electricity now generated by more than 6,000 wind turbines, Denmark produces 80 times as much wind power per capita as Britain.

Why, then, does Denmark export almost all of its wind power - at a revenue loss? Why, then, does Denmark still operate all of its conventional coal-fired power plants? In a phrase, Mr. Sharman says, the reason is Denmark's "wildly fluctuating wind power."

It turns out that Denmark's vast array of turbines often produce minimal electricity when demand is high, maximum electricity when demand is low. Basing his analysis on data from a single year (2002), Mr. Sharman reported that wind power produced less than 1 per cent of the country's electricity supply on 54 different days. On one of these 54 days, the wind turbines took more power from the grid than they produced. (Wind turbines consume considerable electricity whether winds are blowing or not blowing.)

British author and energy analyst Tony Lodge makes the same point in a report by the Centre for Policy Studies, a London think tank. "Not a single conventional power plant has been closed in the period that Danish wind farms have been developed," he says. "Because of the intermittency and variability of the wind, conventional power plants have had to be kept running at full capacity to meet the actual demand for electricity and to provide backup."

Mr. Lodge says it is not practical to turn coal-fired plants off and on as winds rise and fall - because ramping them up consumes more fuel (and emits more carbon dioxide) than running them at a constant rate. Thus Denmark relies almost exclusively on coal-fired plants for its own consumption and exports its wind power at whatever off-peak price it can get.

Only 3.3 per cent of Denmark's wind power gets "accepted" on the grid for domestic consumption. In 2003, Denmark exported 84 per cent of its wind-generated electricity at money-losing rates. And CO{-2}? In 2006, Denmark produced 36 per cent more carbon emissions than the year before.

Denmark has provided generous subsidies for wind power developments since the 1970s and now has a sophisticated wind-dependent industry that - think Vestas - flogs subsidized turbines to naive U.S. presidential candidates. The industry's reliance on subsidies has apparently not lessened in the past 30 years. Mr. Lodge says Danish consumers paid $517-million (U.S.) in wind power subsidies in the first six months of 2007 alone.

Messrs. McCain, Dion and Pickens notwithstanding, winds do not blow predictably. Without an energy storage battery the size of Mount Everest, most wind-powered electricity will be wasted and will almost certainly increase a country's carbon emissions - albeit inadvertently. When your power plant operates at only 20 per cent capacity (or less), you have to build four or five times as many plants as you need. For reliable backup, you still need either coal, gas or nuclear power - all of which are cheaper than wind.

The conclusion seems self-evident. Apparently it isn't. Fortunately, you can test wind power for yourself. Go outside on a hot and humid day. Feel the breeze. Or don't.