21st Century Power Grid Act This bill directs the Department of Energy (DOE) to establish a financial assistance program to carry out eligible projects related to the modernization of the electric grid, including the application of technologies to improve observability, advanced controls, and prediction of system performance on the distribution system and related transmission system inter-dependencies. To be eligible for financial assistance, a project shall: (1) be designed to improve the performance and efficiency of the future electric grid, while ensuring the continued provision of safe, secure, reliable, and affordable power, and provide new options for customer-owned resources; (2) demonstrate secure integration and management of energy resources and secure integration and interoperability of communications and information technologies; and (3) include the participation of a partnership consisting of two or more entities. Each project shall include: (1) the development of a cybersecurity plan written in accordance with guidelines developed by DOE; and (2) a privacy impact assessment that evaluates the project against the five core concepts in DOE's Voluntary Code of Conduct (commonly known as the DataGuard Energy Data Privacy Program), or the most recent revisions to DOE's privacy program.