After more than 5000 h of operation of the 100-kW-Pirot-Electrolyzer developed by the Gesellschaft fiir Hochleistungselektrolyseure zur Wasserstofferzeugung mbH (GHW), the experience accumulated so far allows for the scale up to the Megawatt size of this high pressure electrolysis. Several operational concepts ranging from constant current long time tests over solar photovoltaic energy storage to subsecond compensation of power plant network fluctuations have been successfully simulated. The responses of the main system parameters during these simulations are described. Fdr partial load operation, the normally increased hydrogen percentage in the produced oxygen has been stabilized by a special lye circulation control. The material problems, like corrosion under extreme conditions, are solved up to now. Some of the material aspects are discussed. As measurement and control techniques are the backbone of the safety system, sensors and signal processing are briefly mentioned. The single cell voltage measurement is explained. In total, the gained knowledge allows the design of this kind of electrolyzer for wherever its use is technically and economically attractive. The decreasing acceptance of air pollution improves the position of electrolysis. Beginning with an overview on high-pressure electrolyzers actually realized, an outlook on coming projects and future applications is given.