In an a.c. circuit the e.m.f. ($e$) and the current ($i$) at any instant are given respectively by $e=E_0\sin \omega t$ and $i=I_0\sin (\omega t-\pi /2)$. The average power in the circuit over one cycle of a.c. is:

Average power in an AC circuit is given by:
$$P_{avg}=V_{rms}{I}_{rms}\cos \varphi$$
Where $\varphi$ is the phase difference between voltage and current.From the equations:Voltage phase = $\omega t$ Current phase = $\omega t-\pi /2$ Phase difference $\varphi =\pi /2$.
$$\cos (\pi /2)=0$$
$$P_{avg}=V_{rms}{I}_{rms}\times 0=0$$
This is a purely inductive circuit where the current lags voltage by $90^{\circ }$.