Is energy always conserved?

But I thought it was like stretching water waves. If there is a wave with a wavelength of 1 meter and it is propagating at 1 m/s, then it will take 1 second for all the energy to be absorbed in a wall (ingoring reflection completely here), while it it had a wavelength of 2 meters it would take 2 seconds for the energy to be absorbed. So if an emission lasted 2 seconds, if the wavelength was redshifted to half it's original length, it would seem that the emission lasts 4 seconds, so the power is halved, but the time is doubled, conserving the energy. From what I know the redshifting is just like the doppler effect, where energy is still conserved.