Uninterruptible Power Supply

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What Does Uninterruptible Power Supply Mean?

An uninterruptible power supply (UPS) is an enhanced battery system that will self-activate in the event of a power disruption and function as the primary power source until electronic devices can safely be shut down or an emergency generator takes over.


The purpose of a UPS is to keep power levels consistent and prevent fluctuations that could harm digital or mechanical equipment. All types of UPS systems are designed to keep power levels consistent and prevent fluctuations that could harm digital or mechanical equipment, but there are different design types to provide different levels of protection. The size and capabilities of a UPS system depends on the protected equipment size, which can range from a single computer to a large data center, building complex or city.

Types of UPS systems include:

Offline/Standby: Restores power when normal power fails by using a DC/AC inverter output that is usually no longer than 25 milliseconds. Offline UPS provides battery backup but doesn’t maintain perfect power during sags and surges. This type of UPS is best suited for backing up low power devices, such as small office workstations and personal home computers.

Line-Interactive: Ensures power for five to 30 minutes and up to several hours with expansion, by using a multi-tap, variable-voltage autotransformer, which immediately adds or subtracts the a transformer’s output voltage. Line interactive UPSes can provide moderate protection against power fluctuations, as well as battery backup.

Double-Conversion Online: This is similar to line-interactive, except that a rectifier directly drives a DC/AC inverter, even when it is powered by a normal AC current. This type of UPS protects the critical loads from virtually all power disturbances and is the most common UPS used for protecting large data centers and other critical equipment.

Techopedia Explains Uninterruptible Power Supply

Uninterruptable power sources provide mechanical and digital machines with interim power in a broad range of industries, including manufacturing, information technology, transportation and healthcare.


UPS sytems keep power levels consistent and prevent fluctuations that could cause significant damage to electrical equipment. This is why they play such an important role in managing risk for healthcare, information technology and manufacturing facilities, where keeping electrical systems working continuously is a priority.

It can take time for an emergency generator to take over when there is a power disruption. Even the smallest interruption can have devastating (even life-threatening) consequences in many industries and should be avoided at all costs.

Backup Battery vs. UPS

The terms “backup battery” and “UPS” are sometimes used as synonyms, but they are not the same thing and do not work the same way.

Backup batteries switch on when power fails. Their purpose is allow digital and mechanical equipment to be shut down safely in the event of an outage. Power sags and surges won’t always trigger a battery backup.

Because a UPS continuously monitors incoming power, however, it is able to quickly respond to all types of power interruptions — not just outages.

Some solar-enhanced UPS systems can even be programmed to use their own batteries during peak power times to reduce costs.

Use Cases

UPS systems help ensure business continuity by protecting sensitive and critical equipment from any type of power interruption. This includes providing interim power in the event of:

  • Blackouts – total loss of power
  • Power sags – reductions in voltage magnitude and duration for short periods of time.
  • Power spikes – unexpected, sudden increases in voltage.
  • Over-voltage – voltage that rises above its upper design limit.
  • Under-voltage – voltage that falls below desired limits.
  • Harmonic distortions – undesirable electrical signals caused by voltage and current variations in the electrical distribution system.
  • Noise interference – any undesirable electrical signal that distorts or interferes with desired signals.
  • Frequency fluctuations – short term variations in voltage and current levels.
  • Switching transients – power interruptions caused by abrupt circuit changes.

Monolithic vs. Modular Design

The original design for UPS devices was monolithic. This type of UPS is relatively inexpensive, but components are self-contained and devices have to be scaled vertically (like rack servers) when power demands and redundancy requirements change. This type of UPS is a good option when the damage caused by a power interruption or outage carries low risk.

Modular UPS systems are more expensive, but their components are hot-swappable. This type of UPS can be scaled horizontally (like a blade server) by plugging in additional sub-systems as needed. This type of UPS is a good option when the damage caused by a power interruption or outage carries medium- to high-risk.

Both types of UPS devices use inverters to invoke backup power circuitry when primary power is interrupted. Some modular designs include solar charge controllers to provide an additional layer of redundancy and ensure there will always be a continuous, filtered power flow.

Purchasing Considerations

On the face of it, modular UPS power sources may look more expensive, but, when the total cost of ownership (TCO) is calculated — including ease of maintenance, a smaller floor space footprint and overall system flexibility — a modular approach becomes very compelling.

When making a decision about which type of UPS to purchase, it’s important to first understand what level of protection (and how much redundancy) is required to ensure a critical resource is powered continuously. While traditional monolithic systems typically have a lower initial cost, modular UPS systems can be less expensive to purchase, maintain and repair over the device’s lifecycle.

Ultimately, the choice of whether to purchase a modular UPS instead of a montolithic UPS will depend on how often power demands are expected to fluctuate and the risk of financial and reputational harm should there be a power disruption or outage.


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Margaret Rouse
Technology Expert
Margaret Rouse
Technology Expert

Margaret is an award-winning technical writer and teacher known for her ability to explain complex technical subjects to a non-technical business audience. Over the past twenty years, her IT definitions have been published by Que in an encyclopedia of technology terms and cited in articles by the New York Times, Time Magazine, USA Today, ZDNet, PC Magazine, and Discovery Magazine. She joined Techopedia in 2011. Margaret's idea of a fun day is helping IT and business professionals learn to speak each other’s highly specialized languages.