The "Integrated Resource Planning Modernization Act" aims to enhance the planning and reliability of the nation's electricity system by modernizing integrated resource planning (IRP). It directs the Secretary of Energy, through the Office of Electricity, to undertake three primary actions: develop comprehensive guidelines and best practices for IRP, offer technical assistance to relevant stakeholders, and establish a grant program to support States in updating their IRP frameworks. The Secretary is required to develop these guidelines and best practices in consultation with a broad range of stakeholders, including State public utility commissions, electric utilities, and Transmission Organizations. These guidelines must address critical issues such as iterative capacity expansion modeling, considering diverse alternatives like grid-enhancing technologies and distributed energy resources, and explicitly integrating electric transmission into planning. They also emphasize interregional planning, the integration of other fuel supply systems, and the use of scenario analysis and probabilistic models to account for variability and uncertainty, including extreme weather events. Further requirements for the guidelines include using multiple resource adequacy metrics and scorecards to summarize costs and benefits, encompassing environmental sustainability and economic impacts. Rigorous mechanisms for capacity accreditation for various resources, including conventional, emerging, and demand-side interventions, are also mandated. The Secretary must also provide guidance on how State commissions can review IRPs, promoting public engagement and linking IRP outcomes to regulatory actions like procurement decisions. These guidelines are to be published within two years and periodically evaluated and revised every five years. To support the adoption of these guidelines, the Secretary will provide technical assistance, including workshops and educational materials, to help stakeholders understand and implement the new practices. Additionally, the bill establishes the Integrated Resource Planning Modernization Grants Program , subject to appropriations, to provide financial assistance to States. States can apply for these grants by submitting a plan detailing how they will use the funds to align their IRP with the federal guidelines. For vertically integrated States , grants must be used to develop or update IRP requirements and support utilities in compliance, while restructured States must use grants to coordinate with utilities and Transmission Organizations to implement IRP strategies. Eligible expenses include consultants, software, and stakeholder engagement activities, and the grant amounts will be determined based on factors like existing IRP requirements, the comprehensiveness of the State's plan, and the number of electricity consumers. The Secretary is encouraged to coordinate with other federal programs to maximize the impact of these grants and ensure the guidelines complement existing initiatives for electricity system resilience. Finally, the bill mandates periodic reports to Congress, detailing how States utilized the grants, the expenses paid, and assessing the overall effectiveness of the program in promoting modernized integrated resource planning.