Series Elastic Actuators (SEA) have been in development for multiple decades. In spite of this, few design guidelines exist and stiffness selection for the compliant element still remains a trial-and-error process. In this paper, we experimentally validated the unlumped model first proposed by Orekhov for Rotary SEA (RSEA) and outlined a design methodology for selecting the spring stiffness based on the open loop force control bandwidth of unlumped model for series elastic actuators. We modified the unlumped model to apply to Rotary SEAs. Through experimental system identification, we demonstrated that our new unlumped model for RSEA is a valid model of actuator dynamics. Additionally, we recommended design guidelines for RSEA to achieve desired force control bandwidth based on the pure torque source assumption. An example of the design process was given and actuator performance was verified through dynamic simulations in ADAMS.