Deployment of a battery energy storage system for the photovoltaic (PV) application has been increasing at a fast rate. Depending on the number of power conversion units and their type of connection, the PV-battery system can be classified into DC- and AC-coupled configurations. The number of the components and their electrical loading directly affects the reliability of each of the configurations. Hence, in order to assure high efficiency and lifetime of the PV-battery system, reliability assessment of power conversion units (representing the most reliability-critical system components) is necessary. With respect to that, in this paper, a reliability assessment of the PV-battery system is performed and a comparison of the DC- and AC-coupled configuration reliability is conducted. In the analysis, all parts of the power conversion system, i.e., DC/DC and DC/AC converter units, are taken into consideration and component-, converter- and system-level reliability is assessed. A case study of 6 kW PV system with integrated 3 kW/7.5 kWh battery system has shown that higher reliability is achieved for DC-coupled configuration. The obtained results indicate that the probability of failure for the 15% of the population for DC-coupled configuration occurs 7 years later than that is a case for AC-coupled configuration. Finally, the presented analysis can serve as a benchmark for lifetime and reliability assessment of power conversion units in PV-battery systems for both configuration types. It provides information about differences in electrical and thermal loading of the power conversion units and resulting reliability of the two configurations.