When two waves having the same frequency and a constant phase...

When two coherent waves (same frequency and constant phase difference) interfere, the resultant intensity at any point is given by $I = I_1 + I_2 + 2\sqrt{I_1 I_2}\cos\phi$.

Since the phase difference $\phi$ is constant in time, the intensity $I$ at any given point in space remains constant with time.

The energy is redistributed in space, forming regions of maximum intensity (constructive interference) and minimum intensity (destructive interference).

The total energy of the system remains conserved, meaning there is no overall gain or loss of energy.

Thus, energy is redistributed, and this distribution remains constant in time.