Consider a situation in which a light source is stationary with respect to an observer, A. This source emits n photons of frequency f, each of energy E = hf, towards A. Hence, A will be able to detect the energy loss in the source, which is E = nhf.
Now, this light source is moving with respect to A, and emits n photons of frequency f just like before. However, due to the Doppler Effect, the apparent frequency in which A detects is different, say v. As a result, the energy loss in the source detected at A will be E = nhv, and not E = nhf, which should be the actual energy loss.
How can this be explained? And is this consistent with the wave theory of light?