This paper introduces a generalized reliability model of a solar power unit (SPU) based on physical characteristics including material, operating conditions, and electrical ratings. An SPU includes a photovoltaic panel, power converter, control and sensing. Possible faults in each component of the unit are surveyed and their failure rates based on physics-of-failure models are formulated. PV panel faults include possible installation faults, environmental effects, and material degradation. Power electronics faults are developed in depth for different components of a dc-dc boost converter. A systemlevel simulation model is developed and verified experimentally, and then used to define the survivor function of the SPU. Results show that it is important to include panel faults for accurate reliability values. The developed model is flexible and can be tailored for various SPU operating conditions, panel designs, and electrical ratings. The proposed reliability model can be extended to parallel and series interconnected topologies of multiple SPUs.