Personal UPS Consult

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 elaborately 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, overvoltage’s, 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:




Power Failure

Power Sag

Power Surge



Electrical Line Noise

Frequency Variation

Switching Transient

Harmonic Distortion

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



  • 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



  • 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



  • 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.






Typically large & heavy

Typically small & light

Practical Power Range (kVA)




Voltage Conditioning


Design Dependent


Cost per VA





(typically 95-98%)

(typically 90-96%)

(typically 80-90%)





Typical Application Fields

Homes; Small Offices

Small and medium businesses

Telecoms; Communications; Banking; Transportation; Industrial Environments

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.