At a time of intense debate over the federal budget, government subsidies for wind and solar power are more contentious than ever. The question of whether those subsidies are justified has taken on fresh urgency with the looming expiration of a major wind subsidy.

The federal tax credit for wind-power producers will expire at the end of this year unless Congress extends it. There is widespread agreement that pulling the plug on the subsidy at this point could hobble the wind-power industry. Meanwhile, the biggest federal...