In the last decade a number of large-scale, grid-connected photovoltaic systems have been installed. In order to assess the operating experience and maintenance requirements of these systems, the Electric Power Research Institute (EPRI) sponsored an evaluation of the operation and maintenance (O&M) experience at seven selected photovoltaic installations in the United States. This paper contains the results of the project and includes the presentation of the actual O&M costs experienced by the seven sites, documentation of the largest problems areas, identification of failure causes, and projections of O&M costs for future installations. © 1990 IEEE