We herein investigate the solar photothermal conversion performances of Au nanofluids both experimentally and theoretically. In the experiments, the Au nanofluids are loaded into a beaker and are exposed to solar radiation. The solar photothermal conversion efficiency performances of the Au nanofluids using different nanoparticle (NP) volume fractions and solar intensities are discussed. Experimental results show that the equilibrium temperature increases as the NP volume fraction and solar intensity increase, although the extent of variation in the photothermal conversion efficiency declines under these conditions. Furthermore, a theoretical analysis based on the Rayleigh scattering approximation and the Beer-Lambert Law to predict the solar absorption efficiency is discussed. A good agreement between the calculated absorption efficiency and the experimental solar photothermal conversion efficiency is reported. The absorption efficiency increases exponentially with increasing collector height and NP volume fraction. Moreover, the optimization of NP selection and collector design for direct absorption solar collectors without consideration of the temperature field is of particular scientific importance. (C) 2017 Elsevier Ltd. All rights reserved.