In this study we conceptualise how a capability approach could be used for analysing the conditions for preserving security of energy supply. We derive a concept, 'energy system capability', to describe how well the system is designed for this purpose. Based on a socio-technical systems perspective, we suggest that this capability is composed of five categories of 'building blocks': technical structures, natural resources, economic resources, institutions and actors. The configurations of these building blocks and the interactions between them provide the system with a certain level of reliability, robustness, flexibility, adaptivity and capacity for swift recovery and handling. These building blocks and system characteristics form various capabilities along the event chain (prevent, withstand, recover, handle, prepare, detect etc.) that together build the energy system capability. (C) 2016 Elsevier Ltd. All rights reserved.