Difference Between Online UPS and Offline UPS (With Table)

Uninterruptible Power Supply (UPS) is a system that supplies emergency power to a device when there is a power outage. A UPS normally draws current from the AC mains and supplies it to the load. When there is a power failure, the system switches to using a backup source to draw current. 

Online UPS and offline UPS are two categories of these systems. These act as a backup power supply in case of an electrical failure. 

The difference between online UPS and offline UPS is that an online UPS uses a rectifier and inverter combination circuit to provide power to a device while an online UPS supplies power to the load directly. 

Comparison Table Between Online UPS and Offline UPS

Parameters of ComparisonOnline UPSOffline UPS
OperationAn Online UPS uses a rectifier and inverter combination circuit to carry power from the AC (Alternating Current) mains to the load. An offline UPS directly carries power from the AC mains to the load. 
UtilizationThe inverter is used around the clock, during power availability as well as power outages. An offline UPS inverter is only used when there is a power outage. 
Heat dissipationHeat dissipation is higher than offline UPS as the system is always running. Heat dissipation is lower than online UPS as the system is switched on during electrical failures only. 
Switch and transfer timeThe transfer switch of an online UPS is always on. Thus, the system does not require any time to change the path of current during power outages. The switch of an offline UPS needs to be turned on in case of power outages. It takes about five minutes to switch the path of the current.
DistortionsExternal: An online UPS is resistant to external distortions; Voltage: Voltage distortions do not affect the performance of the system. External: An offline UPS is not resistant to external distortions; Voltage: Voltage fluctuations cause switching delays and affect the performance of the system.
Heat sink requirementAn online system requires a large heat sink. An offline system requires a small heat sink. 
PartsThe design of the system has more components than offline UPS. The system had a lesser number of components than online UPS. 
CostMore expensive than offline UPS. Less costly when compared to online UPS. 

What is Online UPS? 

An online UPS is a type of uninterruptible power supply system that is used as an emergency backup in case of power outages. The system supplies power from the AC mains to the load through a rectifier and inverter combination circuit.

An online UPS is comprised of a rectifier, battery, and inverter. These are fixed in line with the AC mains and the load. The power that is supplied from the AC mains is carried to the rectifier circuit, after which it is converted into DC (direct current) power. 

The converted power is used up to charge the battery. Subsequently, it is transferred to the inverter circuit which supplies the power to the load. 

A transfer switch changes the path of the current in case of a power failure. This switch is always on for an offline UPS. This means that the path of power is changed in no time. Thus, there is no interruption in the power delivered to the load.

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Online UPS

What is Offline UPS?

An offline UPS is another type of uninterruptible power supply system. Unlike an online UPS, this system supplies power directly from the AC mains to the load. In the case of power outages, it uses an inverter to power the AC load from the DC battery.

The output needs to be changed between both the sources using a switch. The transfer time taken by an offline UPS is approximately five minutes. There will be no output of power during this time. There may even be distortions because of the switch of paths. 

During normal operation, the battery of the system is charged when AC power is available. Subsequently, the AC power is supplied directly to the load. 

When there is a power outage, the charged battery supplies DC power to an inverter that converts it into AC power. This AC power is then supplied to the load.

Offline UPS

Main Differences Between Online UPS and Offline UPS

  1. An offline UPS carries the power to a system using a rectifier and inverter combination circuit while an offline UPS carries the power to the load directly. 
  2. An online UPS runs all the time while an offline UPS is switched on only during power outages. 
  3. Since an online UPS system is always running, the heat dissipation is higher as compared to an offline UPS that runs only during power outages. 
  4. An online UPS switches the path of power automatically during a power outage. On the contrary, the path needs to be changed by turning on a switch in the case of offline UPS.
  5. The transfer time taken by an online UPS is zero while the transfer time taken by an offline UPS is about five minutes. 
  6. An online UPS is immune to voltage as well as external distortions. On the other hand, the performance of an offline UPS decreases with constant fluctuations. 
  7. An online UPS is more expensive when compared to an offline UPS. 

Conclusion

An offline UPS and offline UPS are categories of a system that supplies emergency power to a device when there is a power outage. While an offline system transfers the power directly, an online system uses a rectifier and combination circuit to supply power from the AC mains to the load. 

On comparing the two, an online UPS proves to be more efficient. This is why it is mostly used in medical equipment, telecommunication equipment, etc. On the other hand, an offline UPS performs well when used in computers, PCs, and other such devices.

References

  1. https://ieeexplore.ieee.org/abstract/document/1295849/
  2. https://digital-library.theiet.org/content/journals/10.1049/ip-epa_20040020