Due to the rapid increase in energy prices as well as increasing environmental awareness, uninterruptible power supply (UPS) systems are becoming more efficient and more compact. Until recently, on-line UPS systems with isolation transformer have been employed to supply critical loads. However, environment-friendly transformerless UPS systems have been rapidly replacing the old technology due to their superior performance, higher efficiency and smaller size attributes. In this paper, the technology (topology, components, control, etc.) of the environment-friendly transformerless UPS systems with the 6-300kVA range will be discussed and their experimental performance, and size/weight information will be provided. Energy savings via the improved efficiency and positive impact on the eco-environment via the material size/weight/manufacturing process reduction will be emphasized.