In general, the power consumed in mixing operations have been expressed by using power consumption (P) or power number (N-p). These values are the instantaneous ones in the mixing processes. In this work, the change of power from the start to the end of mixing was measured for steady and several unsteady mixing modes, the sum of power per unit volume throughout the mixing process was calculated and the method how to get the optimum mixing conditions to obtain the most economical mode (ECO mode) was proposed.