According to the sources, the average power dissipated in an AC circuit is given by $P = V I \cos \phi$, where $\cos \phi$ is the power factor . In a purely resistive circuit, the voltage and current are in phase (phase angle $\phi = 0$), making the power factor $\cos 0 = 1$. This results in maximum power dissipation, $P = V I$ . In contrast, for purely inductive or capacitive circuits, the phase difference is $\pi/2$, meaning $\cos \phi = 0$ and the average power dissipated is zero .