Difference between Online UPS and Offline UPS

Online UPS and Offline UPS are the two crucial types of UPS. Both supply power to the device in case of power outage. But the similarities end here. The main difference between Online UPS and Offline UPS is that the Online UPS provides the power with the help of rectifier and inverter combination. It simultaneously provides power to load as well as the battery so that battery can be charged to supply power in case of power failure.

On the contrary, an Offline UPS supplies AC power directly to the load by switching ON the transfer switch. And in case of power failure, the offline UPS supplies the power from the battery backup. Another crucial difference between Online UPS and Offline UPS is the requirement of the large heat sink.

The Online UPS consists of the rectifier which provides power directly to the inverter as well as to battery. Thus, the heat dissipation will be large. Therefore, Online UPS require large heat sink in comparison to an Offline UPS.

There exist some other significant differences between Online UPS and Offline UPS; we will discuss it with the help of comparison chart.

Content: Online UPS and Offline UPS

1. Comparison Chart
2. Definition
3. Key Differences
4. Conclusion

Comparison Chart

Parameters	Online UPS	Offline UPS
Definition	An online UPS is one which supplies the AC power through rectifier and inverter combination circuit in case of power availability as well as in power outage.	An offline UPS is one which supplies the AC mains power directly to the load in case of power availability and whenever there is power outage it supplies power to load through backup battery.
Heat dissipation and Operating temperature	Heat dissipation is more in comparison to offline ups because online ups is ON continuously.	Heat dissipation is low because offline ups is switched ON only in case of power outage.
Transfer Time	Time required to switch the path during power outage is termed as transfer time. As the transfer switch in case of online ups is always ON thus, transfer time is zero.	Transfer time is about 5 ms.
External distortions	Online UPS is immune to external distortions such as surges, spikes and power failure.	Offline UPS is not immune to external distortion.
Components	It require more components and thus it is complex to design.	It require less components and thus simpler in design.
Heat Sink	Large heat sink is required in case of online ups.	Small heat sink is required in case of offline ups.
Cost	Costly in comparison to offline ups.	Less costly than online ups.
On-time of Inverter	The inverter is ON in power outage as well as during power availability.	The inverter is ON only when the power failure occurs.
Reliability & efficiency	The online ups is less reliable because it is in ON state always.	The offline UPS is more reliable because it is in ON state only during power outage and thus its efficiency is high.
Applications	In medical equipment, in telecommunication equipement etc.	In computers, PCs etc.

Definition

Online UPS

The Online UPS consist of a rectifier, battery and inverter in-line with AC mains and load. The AC power supplied to the Online UPS is fed first to the rectifier circuit, then it will convert it into DC power. Then, this DC power is used to charge the battery, and this DC power is supplied to the inverter circuit, and then it is supplied to load.

There is transfer switch, but that will always be in ON position. Thus, at every instant of time, the power delivered to the load will be from the combination of rectifier and inverter circuit. The main advantage of using Online UPS is that no switching is required between the main power line path and battery backup path in case of power outage.

Advantages of Online UPS

1. The Online UPS do not require switching between the two paths in case of power failure. Thus, the power delivered to the load will not be interrupted.
2. The significant advantage of using Online UPS is that the isolation between the load circuit and input circuit is proper, which is useful to get distortion less signals.
3. In Online UPS the inverter is always in ON condition. Thus the power delivered to the load will be available at every instant of time, as there is zero transfer time.

Disadvantages of Online UPS

1. The designing of Online UPS is quite complex, as it requires a large heat sink.
2. The power dissipation in case of Online UPS is more because it is supplying power instantaneously to battery backup as well as the inverter circuit.
3. The cost of designing of online ups is much more than that of Offline Ups.

Offline UPS

The Offline UPS directly supply the AC power to the device connected to the load. In case of power failure, the offline UPS changes the position of the transfer switch. And it connects the load to the battery backup path.

The battery of the offline Ups gets charged during the availability of AC power. In that condition, the AC power is directly supplied to the load circuit, and at the same time the battery of the offline ups gets charged with the help of rectifier circuit

In case of power outage, the charged battery is used to supply DC power to the inverter which in turn converts the DC power into AC and supply it to the load terminal.

Advantages of Offline UPS

1. The cost of designing of Offline UPS is very low in comparison to the online UPS.
2. The internal control of offline ups is simple as two paths are there and both works at the different time according to the position of the transfer switch.
3. The efficiency of the offline UPS is high because of the battery backup and inverter in not ON all the time as in case of Online UPS.

Disadvantages of Offline UPS

1. The quality of the electric signal obtained from Offline UPS is very poor because the inverter is not always ON.
2. The transfer time in case of the power outage is about 5ms, during this time there will be no output power. There will be a slight disruption in the output power due to transfer time.
3. The offline Ups directly supply the AC power when it is available, then the spikes and surges if present in the input voltage will also be passed to the output circuit.

Conclusion

The UPS is a device which supplies power to the load circuit without any interruption, not even in case of power failure. The online Ups is the one which supplies the AC mains power to the load circuit via rectifier and inverter circuit and simultaneously supplying power to the battery backup. While the Offline UPS directly supplies the AC power to the load circuit and connect to inverter, rectifier and battery backup only in case of power outage.

Both are crucial device to obtain uninterrupted power. It only depends on our application that which UPS is to be used. If the transfer time of few milliseconds can be tolerated and you are seeking for less costly ups, then offline ups will be the best choice.

But if you are dealing with the application such as medical equipment, industries, telecommunication where even a few millisecond transfer time is not desirable then Online UPS should be used but keep in mind its high cost.