Enel

We expect revolutions to be led by tiny upstarts, not established giants. But Enel, Italy’s $101 billion-in-revenues utility—No. 69 on the Global 500—is charging the barricades when it comes to clean power. Last year it generated 38% of its total output from renewable sources, including wind, solar, and geothermal energy. CEO Francesco Starace says that figure will be 48% in four years, and he has committed to making Enel—whose fastest-growing subsidiary is Green Power, which had $3.9 billion in revenues last year—carbon-neutral by 2050.