A power‐flow theorem which clearly brings out the contribution of various mechanisms to power flow, stored energy, and power dissipation has been derived for acoustic waves in piezoelectric semiconductors. A major feature of the theorem is that it holds for any direction of propagation of acoustic wave with respect to the direction of dc drift velocity of the carriers. Therefore, it is expected to be useful in understanding the effect of bulk‐wave radiation on the performance of surface‐wave interdigital transducers on piezoelectric semiconductors, such as in surface‐wave amplifiers and unidirectional delay lines, where the bulk waves launched are at an angle to the drifting carriers.