Panel a shows the effective area [cm²] as a function of the X-ray energy for the two instruments STIX and GOES. For STIX, the effective area of the two pixels 2 & 5 of the BKG detector (see Fig. 2) is given. Panel b shows the hypothetical spectra STIX BKG and GOES would observe for a flare described by an isothermal model with T = 20 MK. Panel c gives the percentage of high energy photons compared to all photons measured by GOES as a function of the flare temperature [MK]. The three curves show the percentage of the high energy tail of GOES for three different cut-off energies of 4 (dotted), 5 (line) and 6 (dashed) keV.