This paper presents the methodology for determining the actual PQ diagram as a key document for any power generation unit. It describes all the limits of which a hydrogenerator PQ diagram consists. There is a detailed explanation of the theoretical basis of the procedures for their determination. It also contains a reference to the less researched problem of heating in the hydrogenerator end region. The paper clearly shows and explains the sequence of the procedures which make up the methodology. Detailed analyses and calculations, particularly using modern numerical programming tools, provide a good insight in generator performance and may serve as a preparation for testing and measuring. The measurement and testing data is processed and interpreted, and it is only on the basis of the results thus established that the limits for the permitted operation of the generator are positively determined. The final result of the testing conducted in accordance with the methodology proposed is the actual PQ diagram: the real PQ diagram of the hydrogenerator, confirmed by the measurement.