Push through a bulletproof revolving door in a nondescript building in a dreary patch of the former East Berlin and you enter the control center for Vattenfall Europe Transmission, the company that controls northeastern Germany's electrical grid. A monitor displaying a diagram of that grid takes up most of one wall. A series of smaller screens show the real-time output of regional wind turbines and the output that had been predicted the previous day. Germany is the world's largest user of wind energy, with enough turbines to produce 22,250 megawatts of electricity. That's roughly the equivalent of the output from 22 coal plants--enough to meet about 6 percent of Germany's needs. And because Vattenfall's service area produces 41 percent of German wind energy, the control room is a critical proving ground for the grid's ability to handle renewable power.

Like all electrical grids, the one that Vattenfall manages must continually match power production to demand from homes, offices, and factories. The challenge is to maintain a stable power supply while incorporating elec­tricity from a source as erratic as wind. If there's too little wind-generated power, the company's engineers might have to start up fossil-fueled power plants on short notice, an inefficient process. If there's too much, it could overload the system, causing blackouts or forcing plants to shut down.