Making Large UPS Systems More Efficient

by Richard L. Sawyer

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Executive summary

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.


The traditional approach to the specification and selection of UPS systems has focused almost solely on system reliability, as represented by the mean time between failure (MTBF) provided by manufacturers and consulting engineers. Two issues are now conspiring to move efficiency, as much as reliability, to the forefront in UPS evaluation: (1) a focus on total cost of ownership (TCO) over the lifetime of the system, and (2) public and private environmental initiatives, as exemplified by “green building” certification programs and demand-side management programs offered by utility companies.

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