writes "Researchers have carried out an environmental lifecycle assessment of 2-megawatt wind turbines mooted for a large wind farm in the U.S. Pacific Northwest. They conclude that in terms of cumulative energy payback, or the time to produce the amount of energy required of production and installation, a wind turbine with a working life of 20 years will offer a net benefit
within five to eight months of being brought online."
Watts Up With That? has a more skeptical take on the calculations