The Lone Star state famously leads the U.S., itself the world leader, in wind power. But how much wind power-really-does Texas have?

Less than one-tenth of its official tally of more than 8,000 megawatts, says Robert Bryce in the Energy Tribune. That's because wind power is a lot more fickle than other power sources, such as natural gas, coal, or nuclear power.

The Texas electricity authority, ERCOT, figures the state's wind power capacity is only 8.7%. That means for every 100 megawatts installed in a wind farm, power authorities can only count on seeing 8.7 megawatts of electricity produced. That's a lot less than the standard line that wind power in the U.S. produces at about 30% or 35% of its nominal capacity.

Wind power is the biggest source of renewable energy (other than large hydroelectric projects) and it's the great hope of clean-energy advocates. Wind power's success in the heart of the oil patch has been, as Mr. Bryce notes, a talking point for local politicians, the Obama administration, and environmentalists alike. Are things really that grim?

Yes and no. Getting a handle on how much power wind farms actually produce is tricky business. ERCOT itself has danced between estimates at low as 3% and as high as 16% in recent years, before settling-temporarily-on the 8.7% figure. Temporarily, because the Texas Generation Adequacy Task Force is "concerned" with how ERCOT arrived at that figure and still aims to determine "the true capacity value of wind."

The picture isn't much different in the rest of the country. Electricity regulators and utilities have tried to get a handle on how much juice wind power actually produces, and estimates vary widely-from as low as 5% to as high as 30%. Last year's NREL report has all the details.

The biggest problem is in measurement-should you count wind power's production in the summer (not so windy) or winter (windier)? During the afternoon hours of peak demand, or all day? During a single year, or over a several-year period?

The bottom line is that wind power is neither quite the laggard that Mr. Bryce makes it out to be, even in Texas-nor the panacea that many clean-energy advocates hope it will become. Things to keep in mind as the debate over America's clean-energy revolution keeps simmering.