The electron storage ring BESSY was investigated for use as a primary standard of spectral irradiance at wavelengths between 400 nm and 1000 nm. A method was devised to transfer the spectral irradiance scale realized on to tungsten halogen lamps by means of a radiometer that measured either the spectral irradiance of BESSY or that of the lamps. The relative systematic uncertainty of the spectral irradiance produced by the storage ring was determined to be 0.3%. Presently, the calibration of tungsten halogen lamps as radiometric transfer standards of spectral irradiance from synchrotron radiation yields a relative systematic uncertainty of 0.8%, which is below the uncertainty that is currently achieved using black-body radiators. Spectral irradiances of tungsten halogen lamps measured by means of synchrotron radiation and by means of black-body radiation agree within the combined uncertainties of the two methods. We conclude from the investigations that a reduction of the uncertainty of the spectral irradiance of the storage ring to approximately 0.1% can be achieved if it seems worthwhile. Also, the uncertainty in the calibration of the spectral irradiance of tungsten halogen lamps could be reduced below 0.5% if desired.