Electrification of aircraft power and propulsion systems is critical for reduction of aircraft emissions (greenhousegases and acoustic noise). The disruptive nature of electrical propulsion systems for aircraft, and theassociated lack of legacy electrical power system (EPS) solutions presents as an opportunity for new solutionsto optimize the overall performance of these new aircraft. However, the lack of legacy architecture solutions,combined with the increased power levels of hybrid electric aircraft, is a major challenge to the design of EPSto meet performance requirements (reliability, weight, volume, efficiency). Existing approaches to EPS design for aircraft with electrical propulsion (all or hybrid) assume a starting pointwith a comprehensive set of baseline requirements, sufficient to commence informed EPS design. This paperdirectly addresses the challenge of how to determine these baseline requirements for a new concept aircraftwith minimal initial design criteria, and ensure that architectures developed will meet safety requirements. Thisis achieved by translation of expected certification criteria to failure modes, during the occurrence of which flightmust be maintained. From these, baseline requirements for subsequent EPS designs, including system tradesfor optimized EPS architecture solutions and interfaces with non-electrical power systems, are captured.Through a case study for a concept, low emission distributed, hybrid electric propulsion, long aspect wing ratioaircraft, capture of baseline criteria and subsequent EPS design (including system design trades) and interfacesto associated non-EPS systems design is demonstrated.