The photo-to-electricity energy conversion efficiencies of ruthenium-dye-sensitized solar cells (DSC) are measured under a solar simulator. The error in conversion efficiencies was compared under a variety of spectral conditions. Measurements of the conversion efficiencies of DSC between a solar simulator and outdoor sunlight result in about 10% error. This error was seen when the spectral intensity of a xenon-lamp solar simulator (imitating an air mass (AM) 1.5 spectrum) was adjusted by the short-circuit photocurrent I-Si of a crystalline silicon (c-Si) standard cell. In order to adjust the energetic intensity of AM 1.5 for DSC that has a spectrum response only in the visible region light, the c-Si reference cell is modified with a glass UV filter (KG-5, Schott) and the solar simulator was adjusted by IIR-cut si. The energetic spectrum of the solar simulator has a good accuracy over the wavelength range 300-750 nm, giving the conversion efficiency of DSC an accuracy of about 2%. The dependency of the ratio of I-Si to IIR-cut Si on natural sun power is discussed in view of scattering of the visible light under changing natural sun light. (C) 2004 Elsevier B.V. All rights reserved.