DC-to-AC inverter with automatic line-to-battery transfer and integrated charging system serves as an extended run UPS, standalone power source or an automotive inverter suitable for rugged RV applications. It supplies up to 1250W of continuous 120Vac power from any 12V battery or automotive DC source. When hardwire AC input is energized, commercial power passes through to connected equipment and the battery set is recharged via 3-stage, 14/55A charging system. In UPS mode, the APS system responds to blackouts and severe voltage fluctuations with near instantaneous automatic transfer to battery-derived AC output. It includes set of high current DC input terminals for simple installation. It has reliable large transformer design with frequency control powers resistive electronic loads or large inductive motors, compressors and other items with high current needs on start-up. Optional APSRM4 wired remote power switch with full status LEDs provides remote power inverter on/off switching and continuous status information. It supports an unlimited amount of runtime with any number of user-supplied batteries connected. It is highly adaptable to variety of applications and site conditions with adjustable charger settings for wet/gel battery types and selectable line to battery power transfer voltages. Versatile inverter/charger system with auto-transfer switching serves as an automotive inverter for RVs, over-the-road trucking, conversion vans, fleet service vehicles, standalone alternative power source for off-grid, alternative energy or export applications and as an uninterruptible power supply (UPS) for items compatible with 16.6ms transfer time.