Uninterruptible Power Supply (UPS) A device that provides battery backup when the electrical power fails or drops to an unacceptable voltage level. Small UPS systems provide power for a few minutes; enough to power down the computer, while larger systems have enough battery for several hours. In mission-critical datacenters, UPS systems are used for just a few minutes until electrical generators take over.

UPS systems can be set up to alert file servers to shut down when an outage has occurred, and the batteries are running out.

Surge Suppression and Voltage Regulation

A surge protector filters out surges and spikes, and a voltage regulator maintains uniform voltage during a brownout, but a UPS keeps a computer running when there is no electrical power. UPS systems typically provide surge suppression and may provide voltage regulation. See surge suppression.

Standby and Line Interactive

A standby UPS, also called an “offline UPS,” is the most common type of UPS found in a computer or office supply store. It draws current from the AC outlet and switches to battery within a few milliseconds after detecting a power failure.

The line interactive UPS “interacts” with the AC power line to smooth out the waveforms and correct the rise and fall of the voltage.

Online Uninterruptible Power Supply

The online UPS is the most advanced and most costly UPS. The inverter is continuously providing clean power from the battery, and the computer equipment is never receiving power directly from the AC outlet. However, online units contain cooling fans, which do make noise and may require some location planning for the home user or small office.

Offline Uninterruptible Power Supply

Offline Uninterruptible Power Supply also referred to as VFD (Voltage and Frequency Dependent) or Standby UPS offer the most basic level of power protection.

When mains supply is present, the UPS output is supplied via a built-in EMI/RFI filter, which provides the load with protection from spikes and transients by clamping peak voltage to pre-defined levels.

When the mains supply fails or fluctuates outside the UPS’s operating window, a relay connects the load to the inverter output (resulting in a 4-8ms transfer time). In normal operation, with mains supply present, both output voltage and frequency will track the input voltage and frequency respectively.

As the inverter is switched off when the UPS is operating normally, the term ‘Offline’ is given to any UPS of this design. The inverter output on Offline UPS is typically a square-wave.

Offline UPS also referred to as VFD (Voltage and Frequency Dependent) or Standby UPS offer the most basic level of power protection.

When mains supply is present, the UPS output is supplied via a built-in EMI/RFI filter, which provides the load with protection from spikes and transients by clamping peak voltage to pre-defined levels.

When the mains supply fails or fluctuates outside the UPS’s operating window, a relay connects the load to the inverter output (resulting in a 4-8ms transfer time). In normal operation, with mains supply present, both output voltage and frequency will track the input voltage and frequency respectively.

As the inverter is switched off when the UPS is operating normally, the term ‘Offline’ is given to any UPS of this design. The inverter output on Offline UPS is typically a square-wave.

Offline UPS are the most basic models and designed for use in small, non-critical applications that require protection against momentary loss of power. They are used to protecting workstations, terminals, or equipment below 1 kVA.

Typical internal battery autonomy with Offline UPS lasts for just a few minutes, and there doesn’t tend to be the option to add external battery packs for additional autonomy.

It is not advised to use a VFD UPS to protect critical loads or sensitive electrical equipment.