The best modular UPS systems are designed to increase the global system’s reliability and availability with minimum downtime - thanks to their hot-swappable parts and cost-effective redundancy, the complete independence of each module and no fault propagation if one of the modules fails. However, modular UPS systems are made up of several power modules, and it has also been shown that a high number of modules increases the internal failure rate of the system and reduces its reliability. Consequently, to avoid this problem, the MTBF of the modules should be much higher than the typical MTBF of a standard UPS. The aim of this guide is to describe the main design rules to be adopted to reach an enhanced MTBF of at least three times the best standard MTBF of the UPS system. It further explains how to provide a consistent proof of the MTBF, using both preventive estimations and empiric calculations based on real field data for confirmation.