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Business: Industrial


Uninterruptible power supplies

Purchasing Tips

UPS type:

The first decision to make is on the type of protection you require. Offline UPSs offer higher efficiency than online UPSs but provide less protection against disturbances in the power supply. Buy offline units only if the level of protection required by your application warrants the choice and you can tolerate a very brief loss of power during the switchover to UPS power.


Many UPS systems are oversized to accommodate possible future load expansion. The actual installed load may be only a fraction of the UPS power rating. Since the efficiency of UPS systems can drop considerably with decreasing load, cost savings may be significant when the UPS power rating is closely matched to the existing load.

Part-load efficiency:

The efficiency of a UPS is typically reported under full-load conditions. However, for certain applications where the duty cycle of the equipment is low, part-load efficiency becomes important.

Part-load efficiency depends on the type of UPS, the type of load connected and the fraction of rated power. For example, a survey of various UPSs indicated a range of efficiencies from 70 to 94 percent for computer use at rated loads, but only 24 to 66 percent during part-load conditions. Ask suppliers for the part-load characteristics of their equipment that apply to your anticipated load profile.

Efficiency modes:

Some online UPSs can be manually switched to line-interactive mode, increasing efficiency in exchange for lowering protection. This option is worth considering where you can sustain power supply disturbances but periodically need the full protection of an online UPS.

Life expectancy:

UPSs that have a capacity range below 15 kVA are expected to last 10 to 15 years. UPSs larger than 15 kVA are expected to last 20 to 30 years. See "Reliability" below for information on assuring longer life.


The reliability of UPS systems depends greatly on the owner or operator. The three major factors affecting reliability are:

  • System design and space planning - The owner or operator must plan for expansion in load coverage and in physical space for the UPS system. Poor planning may result in poor load matching and power quality problems such as harmonics, voltage spikes and phase separation.

  • Installation and environment - All associated cables and components must be installed correctly, and the UPS must be installed with adequate ventilation for maintenance and operation. Maintaining the manufacturer's specified range of operating temperatures is important for reliability and longer UPS life. The UPS may require a separate, dedicated air-conditioning unit. Consider UPS and air-conditioning noise when locating the unit in or near work areas.

  • Maintenance - Follow the manufacturer's recommended maintenance schedule.

Next: Operation and Maintenance Tips