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The most fundamental figure of merit for a solar collector is its power rating under a standard solar spectrum. For concentrator systems that employ highly efficient multi-junction solar cells, the rating of the panel becomes sensitive to the spectrum of the direct beam sunlight. By considering typical spectral conditions at a measurement site, we show that the power rating can vary by up to 16%. Further, we consider to what extent a single reference spectrum can be used to characterize the irradiance at a particular location. Electrical energy yields produced using both synthesized and standard reference spectra are compared, with standardized references shown to be unsuited for accurate energy yield predictions under realistic spectral conditions.