One of the main concerns in development and operation of mission-critical systems is system security. It is of critical importance to properly specify and implement system security requirements. In this paper, we apply a system security requirements analysis method to elicit, specify, categorize, and prioritize the system security requirements on the smart grid as a complex system-of-systems case study. In particular, we apply the Security Quality Requirements Engineering (SQUARE) method as a representative system security requirements analysis method. The main contributions of this research are the smart grid customer domain security requirements specification and its analysis from the system-of-systems perspective. We elicited a large proportion of system and system-of-systems vulnerabilities and requirements, and used them to specify secure smart grid intersystem interactions.