Difference Between Online UPS and Offline UPS

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.


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

Key Takeaways

  1. Online UPS (Uninterruptible Power Supply) systems provide continuous, uninterrupted power to connected devices by converting AC power to DC power and back to AC power, ensuring a stable and clean power supply even during power fluctuations or outages.
  2. Offline UPS systems, also known as standby UPS, switch to battery backup power only when a power outage or significant voltage fluctuation is detected, resulting in a brief interruption in power supply during the transition.
  3. The primary difference between online and offline UPS systems lies in their power supply method, with online UPS systems providing continuous, uninterrupted power. In contrast, offline UPS systems only switch to battery backup power during power disruptions.

Online UPS vs Offline UPS

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. 

Online ups vs Offline ups

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Comparison Table

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.

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.

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.

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.

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.

Main Differences Between Online UPS and Offline UPS

  1. 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. 
  2. An online UPS is more expensive when compared to an offline UPS. 
  1. https://ieeexplore.ieee.org/abstract/document/1295849/
  2. https://digital-library.theiet.org/content/journals/10.1049/ip-epa_20040020
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