Power is equal to rate of change of electrical potential energy.
P = dU/dt
U = V.dQ
P = d(V.dQ)/dt
P = V.dQ/dt …..(1)
As we know, the current is equal to rate of change of electric charge.
So, I = dQ/dt
Therefore, putting the value of dQ/dt in equation 1, we get;
P = V.I