Momentum boost for MR 2251−178 (red markers) and PG 1126−041 (blue markers) as a function of the outflow velocity. Crosses show the high-speed, sub-pc-scale wind observed in X-ray by Reeves et al. (2013) and Giustini et al. (2011) (see text). Filled and empty squares are used for the kpc-scale ionised outflow from the Hα and [O III] lines, respectively, as derived in this work. Dashed horizontal (dotted diagonal) lines show predictions for a momentum-driven (energy-driven) wind.