Wind energy predictions for different clumping stratifications of selected O-type stars. The smooth wind models have C_c = 1. The numbers refer to those used in Fig. 1 to identify the clumping behavior. Clumping in the outer winds (stratifications 1 through 3) results in an increase of E_kin because of a higher number of effective driving lines. The effect of clumping on the photospheric spectrum (which occurs in stratifications 1, 4, and 5) is temperature dependent; for the 30 000 K (40 000 K) model, it leads to an increase (decrease) in Ṁ. See Sect. 3.1 for a discussion.