What Is UPS Efficiency And How Is It Calculated?

UPS efficiency is based on how much of the original incoming power is needed to operate the UPS. For example, an uninterruptible power supply with a 95% efficiency rating will have 95% of the original input powering the load and connected systems, with the remaining 5% energy “wasted” running the UPS. For a UPS, higher efficiency equates to lower losses of electrical energy in terms of heat output – low efficiency UPS often require more air conditioning to help keep ambient temperatures safe.

Even a 1% or 2% improvement in operating efficiency can add to up substantial energy costs over the full service life of a UPS (i.e. approximately 10 years), particularly for larger systems with higher power ratings. However, in any discussion about UPS efficiency, it’s worth keeping two things in mind:

Different UPS systems have different efficiencies

The same UPS has different efficiency depending on the load level.

The efficiency ratings that UPS manufacturers publish are based on running in online operating mode with a 100% fully-rated load. But as the load reduces, so too does UPS efficiency. As an example, a UPS running at 20-25% load may only be capable of 85% efficiency. Efficiency is particularly important with parallel-redundant installations, as any inefficiencies arising from individual UPSs that are under-loaded will be exacerbated at scale. This can be a major issue with many legacy installations, where UPS often run at less than 50% of their rated capacity. 

How Does ECO Operating Mode Impact UPS Efficiency? Running the UPS in its dedicated energy-saving mode, commonly known as ECO mode, can boost efficiency to 98-99%. It achieves this by in effect operating as a line-interactive UPS, so the load is powered by the bypass line with the inverter inactive but ready to take over if there’s mains supply failure or fluctuation. But while efficiency improves running in ECO mode, it can affect reliability and so should only be used sparingly.