In this paper, the exergy efficiency of a solar photovoltaic thermal (PVT) water collector is investigated experimentally and numerically. An experimental setup of PVT water collector is constructed and its operating parameters are measured. The measured parameters include solar radiation intensity, wind speed, ambient temperature, solar cells temperature, fluid inlet and outlet temperature, open circuit voltage, short circuit current, maximum power point voltage and maximum power point current, respectively. The numerical simulation of PVT water collector is carried out with the use of one-dimensional steady thermal model and four-parameter current voltage model. The various exergy components in PVT system is introduced and a modified equation for exergy efficiency in terms of exergy losses is obtained. Numerical simulation results are in good agreement with the experimental measurements, which are carried out. Finally, parametric studies have been carried out. The comparison between the modified exergy efficiency of the present work and the one given by previous literature shows that the modified exergy efficiency obtained in this paper has not the deficiencies of the exergy efficiency given by the previous literature and it shows exergy loss terms in PVT system directly. (C) 2015 Elsevier Ltd. All rights reserved.