Many newer UPS systems have an energy-saving operating mode known as “eco-mode” or by some other descriptor. Nevertheless, surveys show that few data centers actually use this mode, because of the known or anticipated side-effects. Unfortunately, the marketing materials for these operating modes do not adequately explain the cost / benefit tradeoffs. This paper shows that eco-mode provides a reduction of approximately 2% in data center energy consumption and explains the various limitations and concerns that arise from eco-mode use. Situations where these operating modes are recommended and contraindicated are also described.