Photovoltaic systems with battery storage are analyzed from the perspective that they can operate as a local power island in circumstances of storm-damage or other grid outage. The specific focus is to determine the optimal battery size for a given solar array size, taking into account reasonable day-to-day and seasonal sunlight variations as well as efficiency losses when converting from DC to AC for connection to the grid, or for provision of power during island mode. Three locations in the United States are used as case studies (Newark NJ, Boulder CO, and Tucson AZ). These provide a wide range of sunlight characteristics and illustrate variability factors that will be similar to many locations in continental North America. The analysis of the probability distributions for sunlight brightness then allow for the establishment of a 95% confidence rating for the steady-state power output from a specific combined battery and solar array configuration when faced with a grid interruption. This rating system can be used as a guide for designing systems for future installation. (C) 2014 Elsevier Ltd. All rights reserved.