As energy resources become more scarce and more expensive, electrical efficiency is becoming a more important performance factor in the specification and selection of large UPS systems. There are three subtle but significant factors that can materially affect a company’s cost of operating a UPS system and particularly the electrical bill. Unfortunately, the people who specify systems often fail to recognise these factors, which leads to increased costs to the owner because operational efficiencies are not correctly considered. This paper discusses the common errors and misunderstandings in evaluating UPS efficiency. UPS efficiency curves are explained, compared, and their cost implications quantified.