A model for the bremsstrahlung spectrum is presented in order to cover photon energies below 1.5 keV, since no reliable description for this energy range allows one to take advantage of the information generated by ultra-thin window Si(Li) detectors. Starting from Kramers formula and assessing a large set of experimental spectra, an analytical expression was fitted as a function of atomic number, incidence energy and photon energy. The expression obtained describes properly the bremsstrahlung spectrum for photon energies ranging from 0.25 to 20 keV and atomic numbers between 4 and 83, at beam energies ranging from 5 to 38 keV. Finally, the proposed model is compared to different expressions for the continuum available in the literature.