sentences:  The irregularities on the demand side are usually determined by the varying consumption of electric power by households and the industry, which depend again mostly on the changing weather conditions.In the recent years, the European markets have witnessed a steady rise of price volatility that was caused among other factors, by the augmented use of renewable energy sources - which can be explained by the fact that a larger portion of the overall electricity production in Europe depends nowadays on weather, especially wind power plants.All electricity producing installations have to adhere to their declared schedule (a production plan specifying the volume of their electricity production for every hour of the day).