Why online UPS is expensive?

Dec. 06, 2023

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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

Comparison Chart

ParametersOnline 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 temperatureHeat 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 TimeTime 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 distortionsOnline UPS is immune to external distortions such as surges, spikes and power failure.Offline UPS is not immune to external distortion. ComponentsIt 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. CostCostly in comparison to offline ups.Less costly than online ups. On-time of InverterThe inverter is ON in power outage as well as during power availability.The inverter is ON only when the power failure occurs. Reliability & efficiencyThe 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. ApplicationsIn medical equipment, in telecommunication equipement etc.In computers, PCs etc.

Definition

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

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.
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.
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

The designing of Online UPS is quite complex, as it requires a large heat sink.
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.
The cost of designing of online ups is much more than that of 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

The cost of designing of Offline UPS is very low in comparison to the online UPS.
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.
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

The quality of the electric signal obtained from Offline UPS is very poor because the inverter is not always ON.
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.
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

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.
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.
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.
The cost of the online UPS is more than the offline UPS.
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.

An Uninterruptible Power Supply refers to a power system that provides emergency power to a load when the input power source or mains power fails, regarded as near-instantaneous protection from input power interruptions. The three general categories of modern UPS systems are Line-interactive UPS vs Online UPS vs Offline UPS, which will be illustrated exlaboratly in the following.

Line-interactive UPS vs Online UPS vs Offline UPS: Working Principles

Offline UPS—Entry-level Power Protection

In an off-line ("standby") UPS system, the load is powered directly by the input power, and the backup power circuitry will only be invoked when the utility power fails.

Specifically speaking, the load is fed directly from the raw mains power rather than the inverter output. The energy storage components—battery charger, battery, and inverter are off-line as far as the load is concerned, although the charger and battery still remain connected to the mains power in order to ensure the battery is always fully charged. When the mains power voltage fails or exceeds the limits, the switch will immediately connect the inverter output to the critical load.

Line-interactive UPS—Intermediate Level Power Protection

A line-interactive UPS maintains the inverter in line and redirects the battery's DC current path from the normal charging mode to supplying current when power is lost.

In this smart design, the battery-to-AC power inverter is always connected to the output of the UPS. When the input AC power is normal, the inverter of the UPS is in reverse operation and provides battery charging. Once the input power fails, the transfer switch will open and the power will flow from the battery to the UPS output. This design offers additional filtering and yields reduced switching transients since the inverter is always on and connected to the output.

Online UPS—The Ultimate Power Protection

An online UPS uses a "double conversion" method of accepting AC input, rectifying to DC for passing through the rechargeable battery (or battery strings), then inverting back to 120 V/230 V AC for powering the protected equipment.

In an online (aka double-conversion) UPS, the input AC is charging the backup battery source which provides power to the output inverter, so the failure of the input AC won't cause activation of the transfer switch. That is to say, if a power loss occurs, the rectifier will simply drop out of the circuit and the batteries will keep the power steady and unchanged. No transfer time during the failure. When power is restored, the rectifier will resume carrying most of the load and begin charging the batteries, though the charging current may be limited to prevent the high-power rectifier from overheating the batteries and boiling off the electrolyte.

Line-interactive UPS vs Online UPS vs Offline UPS: Functions

All of the above-mentioned three categories are invented to protect hardware and electrical equipment where an unexpected power disruption may happen. However, influenced by various working principles, their inherent capabilities are different.

Surge/Noise Protection

All the three UPS systems possess surge suppression and line noise filtering functions to shield the equipment from damage caused by lightning, surges, and electromagnetic (EMI/RFI) line noise. Particularly, the online UPS system offers superior protection on account of the double-conversion operation that isolates equipment from problems on the AC line.

Transfer Time to Battery

When an outage occurs, the transfer time varies:

A break in power to a load of typically 2 to 10 milliseconds is inevitable in offline/standby UPS systems.
Line-interactive UPS systems typically transfer from line power to battery-derived power within 2 to 4 milliseconds, faster enough to keep the most power-sensitive equipment operating without interruption.
The online UPS system does not have a transfer time, because the inverter is already supplying the connected equipment load when an outage occurs.

Voltage Regulation

The voltage regulation is crucial, especially for low voltage conditions:

Line-interactive UPS systems use automatic voltage regulation (AVR) to correct abnormal voltages without switching to the battery. When voltage crosses a preset low or high threshold value, this type of UPS will detect and use transformers to boost or lower the voltage by a set amount to return it to the acceptable range.
Online UPS systems adopt a more precise method of voltage regulation: continuous "double-conversion" operation, isolating connected equipment from problems on the AC line, including blackouts, brownouts, overvoltages, harmonic distortion, electrical impulses, and frequency variations.
When not operating from the battery, line-interactive UPS systems typically regulate output within ±8-15% of the nominal voltage (e.g. 120, 208, 230, or 240 volts), whereas online UPS systems typically regulate voltage within ±2-3%.

In fact, there are some common power issues that may occur in daily operations. The table below will present whether the above-mentioned UPS system will protect against the anomalies:

Offline Line-interactive Online Power Failure ✔ ✔ ✔ Power Sag ✔ ✔ ✔ Power Surge ✔ ✔ ✔ Under-Voltage ✔ ✔ Over-Voltage ✔ ✔ Electrical Line Noise ✔ Frequency Variation ✔ Switching Transient ✔ Harmonic Distortion ✔

Line-interactive UPS vs Online UPS vs Offline UPS: Pros & Cons

Offline UPS

Benefits Limitations

High-efficiency (The charger is not constantly on).
User-friendly to operate.
Affordable price.

Uses battery during brownouts, limited or no protection against power irregularities.
The load is continuously exposed to spikes, transients, and any other aberrations coming down the power line, resulting in the risk of loss or damage to sensitive equipment and data.
Finite transfer time from mains power to the inverter when the mains power supply fails.

Line-interactive UPS

Benefits Limitations

High-reliability.
High-efficiency.
Reasonable voltage conditioning.
Lower electricity consumption.
Lower component count.
Lower operating temperatures.

Impractical over 5kVA.
Not protect against all forms of power irregularities.
Do not provide power-factor correction or frequency regulation.
Require frequent battery use in areas of extreme voltage distortion.

Online UPS

Benefits Limitations

Better voltage regulation.
Conversion time from DC to AC is negligible with no gaps in coverage.
No fluctuations in the voltage, indicating stable voltage quality.
The quality of the load voltage is free from distortion.
Near ideal electrical output, highest protection against all power irregularities.

Complex designs requiring a large heat sink.
Higher power dissipation.
The overall efficiency of UPS is reduced (The inverter is always on).
The wattage of the rectifier is increased (It has to supply power to the inverter as well as charge the battery).
Costlier than other UPS systems.

Line-interactive UPS vs Online UPS vs Offline UPS: Applications

The applications of these three topologies with different operating principles vary from small-size residential homes to large-scale data centers.

Due to its higher energy efficiency and economical nature, offline UPS is most commonly-used for households, small offices with low-budgets for power designs, and some fields that have relatively low requirements for power supply quality. Best value for personal computers, and also suitable for printers, scanners, emergency power supplies, and EPABX.

As one of the main choices, line-interactive UPS that meets the demands of high power reliability is ideal for departmental servers, homes, small businesses, and medium enterprises. In some infrastructure challenged areas where the AC line voltage is unstable, fluctuates wildly, or is highly distorted, a line-interactive UPS may need to charge the battery once or twice a day or even more frequently.

The most intelligent online UPS system, regarded as a default solution for providing back-up power and protection to mission-critical equipment and servers at data centers, can also be applied in fields like computer, transportation, banking, securities, communications, medical requiring a long-time power supply. Specifically, for some induction motor drives and similar other motor control applications, intensive care units, medical equipment, sensitive electrical appliances.

Line-interactive UPS vs Online UPS vs Offline UPS: Which to Choose?

The table below summarizes some of the key points discussed before among the three designs.

Offline Line-interactive Online Size Compact Typically large & heavy Typically small & light Practical Power Range (kVA) 0-0.5 0.5-5 5-5000 Voltage Conditioning Low Design Dependent High Cost per VA Low Medium Medium Efficiency High
(typically 95-98%) High
(typically 90-96%) Low-Medium
(typically 80-90%) Cost Low Medium High Typical Application Fields Homes; Small Offices Small and medium businesses Telecoms; Communications; Banking; Transportation; Industrial Environments

When considering a UPS solution, be aware of the UPS type and corresponding level of protection. The key difference among online vs offline vs line-interactive UPS lies in their working principles, which reflects in the diversities in their features, functionalities, benefits & limitations. Furthermore, the internal design of the topology of a UPS will further affect how it will operate in various application environments.