Performance and compliance objectives have become crucial for critical sites. Balancing security, energy and costs with operational objectives is the current main challenge for critical site administrators. While current systems are not integrated enough to provide an overall view of the site performances on such targets, the FUSE-IT project intend to propose a new paradigm: the convergence of monitoring on energy, building and facilities, cyber and physical security, and ICT will leverage critical sites activities and offer new opportunity in the detection of threats or savings. In this paper, we detail the first building block of the FUSE-IT concept which lies in the definition of cross-domain (i.e. on energy, security, ICT and facility) Key Performance Indicators (KPIs). These KPIs are the main decision variables for an enhanced Decision Support System (DSS) meeting the next century impediments of critical sites.