A rack-mounted natural-cooling RMN uninterruptible power supply (UPS) having ultra-wide-efficient load range, 96% efficiency from 10% load, is proposed. It is based on paralleling multiple multilevel converter modules on an efficient cooling base plate. A load-based active module control (AMC) is proposed to determine the suitable number of active module(s) to achieve high efficiency across the entire load range. By using the new design of heat pipe and heatsink integration and together with high operation efficiency, the proposed UPS can be running without forced-air cooling. A 10 kW, 220 V/50 Hz experimental prototype has been built and evaluated.