Comparison between Online and Offline Uninterruptible Power Supply (UPS)

hat is the Difference between Online & Offline UPS? What are the advantages & Disadvantages of them?

Comparison between Online and Offline Uninterruptible Power Supply

This post gives the details about Advantages, Disadvantages and Applications of the Online UPS and Offline UPS. In addition to that it gives the comparison between the Online UPS and Offline UPS.
Before proceeding further, it is highly recommended to understand the basics of Uninterruptible Power Supply (UPS) and various types of UPS.
Click here to know about the Online and Offline UPS basics.

Online UPS Systems:
A separate post is published about the Online UPS, which gives complete understanding about this kind UPS. Please refer the particular post.
The online UPS Systems have much more complex designs than Offline UPS systems, while the price, weight, and volume are higher. The inverter of an online UPS supplies continuous power to the critical load. Under conditions of overload or loads with high inrush currents which is beyond the capacity of the inverter, the static bypass switch provides mains power to the load. These kind UPS prevent the load from surge voltage, transient voltage, noise and brownouts.

Offline UPS Systems:
This is the simplest form of UPS system. Referring this system as a UPS is not correct because the inverter is normally off. For this reason off-line UPS are also known as standby power sources. The most important advantage of the offline UPS system is low-cost.

Advantages of Online UPS:

  • It provides isolation between main supply and load.
  • Since inverter is always ON, the quality of load voltage is free from distortion.
  • All the disturbances of supply such as blackout, brownouts, spikes etc are absent in the output.
  • Voltage regulation is better
  • Transfer time is practically zero since inverter is always ON.

Disadvantages of Online UPS:

  • Overall efficiency of UPS is reduced since inverter is always ON.
  • The wattage of the rectifier is increased since it has to supply power to inverter as well as charge battery.
  • Online UPS is costlier than other Uninterruptible Power Supply Systems.

Applications of Online UPS:

  1. Induction motor drives and similar other motor control applications.
  2. Intensive care units, medical equipments.

Advantages of Offline UPS:

  • Offline UPS has high efficiencies, since charger is not continuously on.
  • The power handling capacity of charger is reduced.
  • Offline UPS are not very costly.
  • Internal control is simpler in offline Uninterruptible Power Supply.

Disadvantages of Offline UPS:

  • Since offline UPS provides mains supply when it is present, the output contains voltage spikes, brownouts, blackouts.
  • There is finite transfer time from mains to inverter when mains supply fails.
  • Output of offline Uninterruptible Power Supply is not perfectly reliable.

Applications of Offline UPS:

  1. Computers, printers, scanners etc use offline UPS.
  2. Emergency power supplies, EPABX.

Comparison between Online and Offline UPS:

Sl No parameter Online UPS Offline UPS
1  Inverter  Always ON  Turned On when  mains fails
2  Rectifier cum  charger  Supplies power to inverter as well  as charges battery  Charges only  battery
3  Output  waveform  Sine wave  Quasi square wave
4  Harmonic  distortion  Low  High
5  Efficiency  Low  High
6  Load  Isolated from supply  Not isolated from  Supply
7  Cost  High  Low

Read More:
Thyristor: Turn-Off(Commutation) methods
SMPS Advantages, Disadvantages & Comparison with Linear Power Supply
Various SCR Protections

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