The energy of a photon is defined as E = (hc)/λ where h is the planck constant, c is the speed of light, and λ is the wavelength. As a redshift is the increa

The energy of a photon is defined as E = (hc)/λ where h is the planck constant, c is the speed of light, and λ is the wavelength. As a redshift is the increase in wavelength it is an actual decrease in the energy of the photon. This makes more sense when you consider that a photon is mass-less, and so its total, energy is bound up in its momentum rather than momentum and mass.