  A photon induced shower at $E_{prim}\ge 10^{18}$ eV exhibits very specific
features and is different from a hadronic one. At such energies, the LPM effect
delays in average the first interactions of the photon in the atmosphere and
hence slows down the whole shower development. They also have a smaller muonic
content than hadronic ones. The response of a surface detector such as that of
the Auger Observatory to these specific showers is thus different and has to be
accounted for in order to enable potential photon candidates reconstruction
correctly. The energy reconstruction in particular has to be adapted to the
late development of photon showers. We propose in this article a method for the
reconstruction of the energy of photon showers with a surface detector. The key
feature of this method is to rely explicitly on the development stage of the
shower. This approach leads to very satisfactory results ($\simeq 20%$). At
even higher energies ($5.10^{19}$ eV and above) the probability for the photon
to convert into a pair of e$^+$e$^-$ in the geomagnetic field becomes non
negligible and requires a different function to evaluate the energy with the
proposed method. We propose several approaches to deal with this issue in the
scope of the establishment of an upper bound on the photon fraction in UHECR.
