Flywheel energy storage systems have received renewed attention due to their advantages, such as high-power density, long cycle life, and environmentally friendly characteristics. Energy is stored in a flywheel as kinetic energy by spinning a rotor at high speeds. In this context, a specific configuration of a single-rotor flywheel system with a capacity of 500 kWh was constructed in Boxberg/O.L., Germany. This demonstrator features a 42-tonne rotor, delivers 500 kW of power, and operates at a maximum speed of 3000 min-1. This study aims to evaluate the system's performance by analyzing its components and inherent losses, which are calculated using analytical methods. The demonstrator, along with measurement data, serves as the basis for validating the mathematical model. The results highlight the potential and the challenges of largescale flywheels, indicating that a vacuum system, effective sealing, and appropriate bearings can reduce the power losses of 370.5 kW at 1700 min-1 by about 71%.