The present invention relates to a method and device of evaluating a traffic shaper used for shaping at least two traffic flows of a mobile terminal operating according to a particular DRX setting in a mobile communications network. The method comprises the steps of acquiring an estimated lower bound of battery savings as a function of transmission rate of each traffic flow (f1, f2) of the at least two traffic flows for the mobile terminal according to the particular DRX setting. Further, the method comprises acquiring an estimated battery savings measure for said each traffic flow (f1, f2) before shaping, and acquiring an estimated battery savings measure for said each traffic flow (f1, f2) after shaping. Moreover, the method comprises calculating normalized traffic shaper gain as a relation between the estimated battery savings measure for the shaped traffic flow and the estimated battery savings measure for the unshaped traffic flow for said each traffic flow (f1, f2), said relation further being normalized with the respective estimated lower bound of battery savings, and determining whether to use the traffic shaper (S1) for said at least two traffic flows (f1, f2) on the basis of the calculated normalized traffic shaper gain.