If cars have to meet energy standards, why don't power plants? Carbon-emission standards limiting the amount of CO2 that a new power plant can spew are in place in a handful of states. California's tough new rules virtually exclude new coal plants until clean-coal technology comes on line, and could establish a national standardjust as they might for auto emissions. A federal carbon standard would be aggressively opposed by power companies that depend on coal. But it could also spur investment in renewables, clean coal and even nuclear (that's another fight) more rapidly than carbon taxes or cap-and-trade systems. With 159 new coal-powered plants slated for the next decade, a critical choice is looming.