This study addresses energy security from the perspective of comprehensive national defence, especially the interaction between military and civilian aspects of defence. Civilian infrastructure is seldom configured or developed with defence in focus. Therefore, with the aim of facilitating a system-level discussion, this study takes two steps. The first step is to develop indicators for assessing energy security in a comprehensive national defence setting. The second step is to qualitatively assess the effect on energy security from three different energy development scenarios related to either the development of local solar, wind, and bio-power production and storage; an increased resilience of the high voltage electric power transmission system; or an increase in large hydropower and nuclear power production. The study identifies that there are positive systemic effects of increasing the diversity of the energy system, especially for solutions that do not need external supply and do not risk creating large-scale effects if attacked. However, these changes to the energy system also lead to local changes that affect warfighting and defence. Such changes can be met by updated tactics and technology that would also give the defending force an advantage.