Single Phase Uninterruptible Power Supplies (UPS) Information

Single phase UPSSingle phase uninterruptible power supplies (UPSs) sit between an AC outlet (i.e., a wall outlet or power strip) and an electronic device (such as a computer, server, or phone equipment) to provide power conditioning, back-up protection, and distribution for electronic equipment loads. Single phase UPSs also prevent power disturbances (outages, sags, surges, spikes, noise, etc.) from affecting the performance and life of the electronic device and vital data.


Specifications for single phase UPSs include volt-amp rating, watt rating, and input voltage range.

  • The volt-amp (VA) rating is the maximum of volts multiplied by the number of amperes (amps) that a unit can deliver. Note that the VA rating is not the same as the equipment’s power drain, which is measured in watts (W).
  • The watt rating, which is less than or equal to the VA rating, is specified only if the number of volt-amps is unknown.
  • The input voltage range, the precise identification of the electrical system, is critical to the proper selection of single phase UPSs.


There are several different unit types for single phase UPSs. Choices include:

  • online or double conversion
  • line-interactive
  • hybrid or ferroresonant
  • off-line or standby

With an online UPS, the load is supplied from a power converter that operates continuously and takes its input from a DC power supply. This DC supply consists of a battery and large battery charger, which are connected in parallel. The inverter works in parallel to supply AC power to the load and only handles the full-load power if the AC input fails. With a hybrid UPS, the supply conditions power using a ferroresonant transformer. This transformer maintains a constant output voltage even with a varying input voltage and provides good protection against line noise. Off-line or standby UPSs are also available.


Parameters to consider when specifying single phase UPSs include:

  • performance features
  • interfaces
  • surge suppression options
  • general features
  • environmental operating conditions

Performance features for single phase UPSs include self-diagnostics, automatic shutdown, automated saving, and application software. Interface choices include serial, parallel, digital, modem, Ethernet, and PC card. Surge suppression options include UL 1449 rated surge protection, rated AC energy absorption, and maximum surge current.

Related Information

CR4 Community—Designing or Sizing a Single Phase (50 Hz) UPS System

CR4 Community—UPS Circuits with Single Wire

CR4 Community—UPS Output Voltage

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