Silicon carbide is an interesting candidate for high-temperature solar receiver due to its high solar absorptivity and its resistance to oxidation in air. Its oxidation stays passive with the formation of a protective silica layer up to 1900 K. Nevertheless, silica presents a lower spectral emissivity than silicon carbide in the solar range. Therefore the growth of a silica layer may affect the values of the spectral emissivity and therefore the surface receiver thermal efficiency. We have investigated here how high-temperature ageing in air affects the spectral emissivity and the thermal efficiency of commercial Si-SiC materials. We were able to correlate the variations of the spectral emissivities to the advancement of the oxidation using scanning electron microscopy, ellipsometry and 3D profilometry. Especially we observed a significant improvement of the surface receiver thermal efficiency due to the increase of the surface roughness that increases the solar absorptivity.