The smart grid paradigm envisages the wide penetration of distributed energy resources, such as demand-side response (DSR) schemes and energy storage (ES). Despite their potential to improve security of supply at the distribution level, existing design standards in most jurisdictions consider solely conventional assets; conceptual and methodological gaps prevent DSR and ES from being embedded into formal network design practices. As such, the crucial question that arises is how to assess the security contribution of these technologies so as to level the playing field and encourage the transition to a smart grid. Here, the authors introduce two capacity metrics: equivalent firm capacity and equivalent load-carrying capability. The authors describe their application to DSR and ES, showcase results from the UK Power Networks' Smarter Network Storage and Low Carbon London projects, and provide suggestions on the incorporation of smart assets in future design standards.