The CCD camera-based flux measurement at Paul Scherrer Institute’s (PSI) high flux solar simulator (HFSS) is influenced by a spatially variable spectrum of the concentrated radiation characteristic for arc lamp-based solar simulators. This results in a substantial error in the radiative power determined by integration of the flux distribution. This systematic error is assessed by numerically modeling the response of the CCD camera in use. Measured spectra of concentrated radiation obtained at different points in the flux distribution, measured transmission characteristics of all optical elements, and published data for the spectral sensitivity of the CCD chip are applied in the model. The response of a water calorimeter is used as baseline case. It is shown that the magnitude of the error depends strongly on the region analyzed, i.e., on aperture size, on the wavelength band analyzed, and, unfortunately, also on the number of lamps in operation. A relative error in the range of 10–30% is observed if an aperture with 1 cm in diameter covering the region of peak concentration is considered. It will be shown that the error arises due to the fact that a photon counter (CCD camera) is used to determine the thermal power.