A mechanical uninterruptible power supply (UPS) electrical storage conversion system for single-phase residential applications that enables tight integration of storage, sources, and loads is developed and designed. The mechanical UPS, named a rotating transformer, is a permanent magnet transverse flux machine with multiple windings to support multiport conversion between AC/DC loads and sources. It is designed to mitigate second harmonic power ripple, provide voltage conversion and galvanic isolation, and supply hold-up energy. This paper focuses on the rotating transformer's electromagnetic and mechanical design. A prototype was built and preliminary results are presented.