The United States Laboratory Module (U.S. Lab) on the International Space Station (ISS) provides equipment for research and technology development. The ISS Electric Power System (EPS) consists of power generation and power distribution. Primary power is generated (~160 Vdc) by photovoltaic arrays during sunlight and by rechargable batteries during eclipse. Primary power is converted to secondary power (~120 Vdc) by Dc-to-Dc Converters (DDCUs) and is distributed through a series of solid-state switchgear. The U.S. Lab receives power through a set of DDCUs and the power is distributed to the loads through solid-state relays known as Remote Power Controllers (RPCs). This paper presents the results of the U.S. Lab power quality testing and a description of how compliance with specifications and standards is verified. The testing includes measurements of source and load impedances, noise and ripple voltages, and transients due to load steps among others. The paper outlines the challenges involved in the testing and presents an analysis of the results. In addition, the paper addresses the methods used to verify system stability and how the challenge of verifying a large system for stability and power quality performance with limited time and access points was accomplished.