Abstract
This paper specifies and compares the ancillary services used by transmission system operators in selected countries within ENTSO-E (European Network of Transmission System Operators-Electricity) for operational needs of their transmission systems. The comparison of the ancillary services’ unit prices is also given. In addition, analyses of various experiences gained by ENTSO-E countries within their electric power systems in relation to initiatives supporting the power generation from renewable energy sources (RES) and the methods of their integration into the electric power system were made. Namely, depending on the total amount of RES, depending on the incentive system for electricity production from RES and the development of an organized electricity market, different balancing responsibility models for electricity producers from RES were established.