While the light turns on and the TV starts at the push of a button, there lies a tremendous amount of effort behind a secure supply of power. Because of the demand fluctuations, specialists must calculate precisely how much power is needed and when. Weekends, for instance, when offices and stores are closed, require less energy than Mondays. And when it is gray and rainy outside, the lights get turned on earlier than on sunny days. More than anyone else, transmission grid operators have to know in time when electricity from renewable energies is being generated because they have to transport it across the country and partly market it by themselves.