What does online UPS mean?

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.

Difference Between Online UPS and Offline UPS – Which One is Better?

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Main Difference between Online UPS and Offline UPS – Which One to Use?

UPS, Uninterruptible Power Supply is a device that supplies power in case of power failures. A UPS can be either Online UPS or Offline UPS. Both types of UPS can be used as a backup power source but there are many differences between them.

The major point that differentiates between the online UPS and offline UPS is that the online UPS supplies power from the AC mains to the Load through the rectifier and inverter combination while the Offline UPS directly supplies power from AC mains to the load.

Before going to the list of differences between Online UPS and Offline UPS, let’s discuss the basics of Online UPS and Offline UPS.

What is UPS (Uninterruptible Power Supply)?

UPS is an abbreviation for Uninterruptible Power Supply and the reason for its name is that it provides a constant supply of power without any interruption. In Normal operation, it draws current from the AC mains and during a power outage; it draws current from its backup source.

A UPS system utilizes a DC power stored in the battery as its backup source. The UPS charges the battery from the AC mains through a Rectifier and charge controller circuit. At the same time, it supplies power to the output AC load. In case of power failure, the UPS utilizes this charged DC power in the battery and converts it into AC using an inverter and supplies it to the AC load.

Now the switch over between the sources (AC mains and the battery in case of power failure) is instantaneous. But it does take some time. This time delay during the switching may affect some sensitive electronic equipment while other devices may tolerate it in a specific range. This is where Online UPS and Offline UPS comes into play.

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What is Online UPS?

The Online UPS is a type of UPS that supplies power to the AC load through the Rectifier and Inverter Combo in normal operation and uses an inverter to supply AC power during a power failure. Therefore, the output power supply always stays ON and there is no need for switching. Hence, there is no time delay in switching between its sources. There is no interruption in the case of power failure even for a nanosecond.

During normal operation, it converts the AC supply from the AC mains into DC using a rectifier and charges the battery using a charge controller circuit. Now at the same time, the DC power from the charged battery is being converted into AC using an inverter to supply it to the output AC load.

In case of power failure, the UPS will stop drawing current from the AC mains, and stops charging the battery. But the AC load will continuously draw current from the battery through Inverter.

Therefore, Online UPS has a zero time delay when switching its power source. It is also why it is known as Online UPS because it stays ON even during its normal operation.

Since the current drawn by the AC load is continuously flowing through its whole circuit, the temperature of the system increases. Therefore, it needs comparatively larger heat sinks and the components that can withstand high temperature and it can tolerate current flow for very long duration. Due to such a requirement, the cost of Online UPS increases significantly.

It continuously draws current from the battery which quickly reduces the lifetime of the battery compared to offline UPS. They are more inefficient because they continuously stay ON even in normal operation.

On the other hand, it does offer high-quality output because there is no gap during the switching between the source supplies. It completely isolates the output from the AC mains. Therefore, it helps in protection against the distortions or fluctuations in the AC mains.

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What is Offline UPS?

The offline UPS is a type of UPS that directly supplies the power to the AC load from the AC mains in normal operation and uses an inverter to power the AC load from the DC battery. Since there are two separate lines of supplies, the output supply must be switched between the two sources.

The switching speed of an Offline UPS is around 5ms which is enough to keep a computer from shutting down. But not fast enough to keep a smooth operation of sensitive equipment in hospitals.

During normal operation, the power from AC mains is directly supplied to the output through a static transfer switch. At the same time, the AC is converted into DC using a rectifier to charge the battery using a charge controller. But unlike Online UPS, the inverter does not draw current from the battery until the primary supply (AC main) is interrupted.

When the primary supply breaks, the battery stops charging and the static switch transfer the output line to the inverter side very quickly. At the same time, the inverter starts drawing current from the battery which is then supplied to the AC load.

Since it stays off when it draws current from the AC line during normal operation, it is known as Offline UPS.

The Offline UPS mostly stays off in normal operation and because of that, the temperature of the components stays low. Therefore, it requires smaller heat sinks and the components with a lower tolerance can be used which also decreases the overall cost of the offline UPS.

Since it stays off in its normal operation and does not use the battery continuously, the efficiency of the system increases.

But the downside of offline UPS is that the directly supplied AC is not completely isolated from the load. Therefore, there is no protection between the load and source and the distortion or fluctuations in the Ac mains can damage the connected load.

Differences between Online UPS and Offline UPS

ONLINE UPS

OFFLINE UPS

The UPS that supplies power from the AC mains to the load through the rectifier and inverter combination

The UPS that supply the AC mains power directly to the load.

It continuously draws current from the battery.

It only draws current from the battery during primary source failure or power outage.

It requires a large battery charger circuit because it carries the entire load current.

It requires a small charger circuit because it only charges the battery.

The input supply and the load is completely isolated.

There is no isolation between the input supply and load.

It has high-performance output because it protects the output load from input voltage spike and distortion.

Its performance reduces with the input voltage distortion; which is directly connected to the load.

There is not switching between the source, hence no time delay

It uses a static transfer switch to switch between its source with a time delay of 5ms

It stays ON almost at all times.

It turns ON only at power failure.

Due to its continuous ON condition, it is inefficient and unreliable.

It is more efficient and reliable because it mostly stays off.

It requires large heat sinks because of the high operating temperature

It requires smaller heat sinks because of low operating temperature due to its off state mostly.

The components used must have the tolerance to withstand high temperature

There is no need for the components to have high-temperature tolerance.

It is expensive than Offline UPS.

It is cheaper than Online UPS.

It is used for very sensitive electronics that cannot tolerate any gap between supplies such as in-hospital ICUs

Offline UPS is used for electronic equipment that can tolerate a delay of 5ms such as Computer etc

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Which one to Use, Online or Offline UPS & Why?

Is online or offline UPS Better? and When to use it? it totally depends on multiple factors such as cost, usage and specific conditions etc. For example,

When to Use Online UPS:

• When to operate the sensitive and important devices and equipment with continuous power supply without switching operations.

• When the the main power supply fluctuates frequently or daily scheduled load shading and power outage

• When there is no concerns about the operation, installation and maintenance cost as the online UPS system is costly than offline UPS system due to the additional parts in online uninterruptible power supply.

• When you need the simultaneous operation of connected devices for longer time duration.

When to Use Offline UPS:

• When cost is the major and considerable factor.

• When switching is not a big issue i.e. you are OK to use it with auto or manual change-over or transfer switch between main power, portable generator or emergency generator set , solar or battery power.

The conclusion of this article showing the key difference between ONLINE & OFFLINE UPS systems is that the online UPS has zero time delay between the switching and they are expensive. while the Offline UPS has a time delay of 5ms but they are inexpensive. Based on efficiency, the offline UPS is far better than Online UPS and it should be used for domestic purposes. The Online UPS should be only used for very sensitive electronic equipment.

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