Mean energy of the detected photons of GOES/XRS and STIX against the temperature of the isothermal model. Red and blue curves show the mean energies of the GOES 1-8 Å and 0.5-4 Å bands, respectively, whereas the black line shows the STIX imaging mean energy. For all temperatures, we assumed a constant emission measure of 10⁴⁷ cm⁻³. In this case, we do not make any distinction between the STIX imaging and spectroscopy mean energies since, for the considered flare spectra, there is no difference.