The STEREO experiment is measuring the antineutrino flux emitted by the reactor of the Institut Laue-Langevin (ILL) in Grenoble-France. About 400 antineutrinos per effective day of data taking are currently detected. The target volume, segmented in 6 detector cells, allows for a search of a light sterile neutrino in a way independent of the reactor parameters (see poster by P. Del Amo Sanchez). However the detected spectrum also provides a new reference of antineutrino fission-spectrum, with virtually pure contribution from the 235U isotope given the very high enrichment of the nuclear fuel. In this poster we will present the calculations performed to compare this upcoming new experimental reference with a predicted spectrum. This includes the determination of the accuracy of the thermal power, the estimation of off-equilibrium effects for typical 50-days long cycles and the neutrinos emitted by the spent fuel stored on site and activated materials. Finally the normalization of the reference beta fission-spectra measured at the ILL in the 80’s will be discussed.