Difference between Online UPS and Offline UPS

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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.
Key Differences between Online UPS and Offline UPS
  1. The key difference between Online UPS and Offline UPS lies in their working. An Online UPS supplies the AC power through a rectifier and inverter circuit even when the AC mains power is available. On the contrary, the offline ups directly supply the AC mains power to the load circuit when the power supply is available.
  2. The transfer time is the key term which differentiates Online UPS and offline UPS. The transfer time in case of Online UPS is zero as there is no switching between the inverter rectifier path and mains supply path. While Offline Ups possess transfer time of about 5ms.
  3. The other significant difference between Online UPS and Offline UPS is that the inverter in online ups is ON all the time while the inverter in offline UPS is on only when the power supply is unavailable.
  4. The cost of the online UPS is more than the offline UPS.
  5. The efficiency of the offline UPS is more than the efficiency of online UPS because the inverter and the rectifier circuit is not ON all the time in offline UPS.
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.