Department of Defense (DoD) installations rely on the commercial electricity grid for 99 percent of their electricity needs, but extensive energy delivery outages in 2012 have reinforced that the U.S. electricity grid is vulnerable to disruptions from natural hazards and actor-induced outages, such as physical or cyber attacks. In the event of a catastrophic disaster--such as a severe hurricane, massive earthquake, or large-scale terrorist attack--DoD installations would also serve as a base for emergency services. To enhance energy security, DoD has identified diversifying energy sources and increasing efficiency in DoD operations as critical goals. But how to enhance energy security across the portfolio of installations is not clear and several questions remain unanswered: Energy security for how long? Under what conditions? At what cost? The underlying analytical questions are, what critical capabilities do U.S. installations provide, and how can DoD maintain these capabilities during an energy services disruption in the most cost-effective manner? Answering these questions requires a systems approach that incorporates technological, economic, and operational uncertainties. Using portfolio analysis methods for assessing capability options, this paper presents a framework to evaluate choices among energy security strategies for DoD installations. This framework evaluates whether existing or proposed installation energy security strategies enhance DoD capabilities and evaluates strategy cost-effectiveness.