This article presents issues related to the broadly understood development of wind energy. The state of the art of wind energy in the world has been discussed. This paper also shows issues related to factors affecting the development of renewable energy sources and their impact on the natural environment on the basis of available research. The focus is, however, on the critical analysis of phenomena in the data of the Transmission System Operator (TSO) in the period from 1 January 2019 to 31 December 2019. This analysis concentrates primarily on capturing the correlation between demand for TSO power capacity and the electrical power output generated by wind farms. As part of the article, specific operating condition for the TSO has been presented, in particular from the point of view of generating energy by wind sources. These were the days of 2019, in which there occurred: the maximum national demand for electrical capacity of 26,135.6 MW, the minimum national demand for capacity of 11,399.6 MW, the maximum generation of power output from wind farms of 5,222.1 MW, the minimum generation of power output from wind farms of 8.18 MW and maximum hourly variability of wind farm generation - a decrease by 954 MW. The presented work closes with a forecast indicating the need to invest in making conventional generating units more flexible as well as in increasing the system’s installed capacity. The article does not take up issues related to power system security.