“Environmental protection”, “high efficiency” and “energy savings” are nowadays trendy topics and as they also apply to UPS, UPS have become a trendy topic as well. ECO mode represents an interesting feature in UPS technology in particular related to small or single-phase applications with less than 10 kVA. UPS industry has spread its use in the last few years for applications using on-line double conversion products. These products, characterized by double power conversion stages, usually lacked high efficiency performance during on line operation mode, which is why the use of ECO mode brought an extra value to achieve higher efficiency and energy savings. However there is a former ECO mode used inherently in other UPS products and perhaps no so well known: this is exactly the topic we are going to deal with in this paper.