The energy quality of solar energy assisted biogas plants established in Izmir, Turkey was investigated in this study via exergy analysis. This study was assisted together with other energy and economy analysis of these plants. Firstly, the plant was operated at mesophilic temperature region and it was continuously fed with cattle manure. The biogas efficiency reached a value as high as 1.3 m(3)-biogas/m(3)-reactor/day. The exergy efficiency of the entire biogas plant was found in the range of 35-75% and the fluctuations seems to the collector's. The biogas reactor is the unit that has the highest exergy efficiency within the plant. The heat loss from the pipes affected the exergy efficiency of heat exchanger especially, and it indicated that the isolation thickness should be increased. Additionally, the change in the volume of heat exchanger and/or in the number of collector was suggested according to life cycle cost analysis.