What Is a UPS? How an Uninterruptible Power Supply Works

By: HowStuffWorks.com Contributors  | 
An uninterruptible power supply is something you might not think about until you really need it. MF3d / Getty Images

It's a foundational element of device integrity, but you wouldn't be the first person to beg the question, "What is a UPS?"

In short, a UPS safeguards electronic devices from power disturbances and instability. But, of course, there's a lot more to it than that.


What Is a UPS?

A UPS, or an uninterruptible power supply system, is an electrical device designed to provide emergency power to a load when the input power source fails.

Not to be confused with an auxiliary or emergency power system, a UPS provides near instantaneous protection from input power outages via battery power [source: USAID].


The primary role of a UPS is to bridge the gap between a power loss and the activation of a longer-term backup power system, like a generator.

The Mechanics Behind UPS Technology

From the power grid, a computer expects to get (in the United States) 120-volt AC power oscillating at 60 Hertz (see How Power Distribution Grids Work for more information). A computer can tolerate slight differences from this specification, but a significant deviation can cause the computer's power supply to fail.

This is where uninterruptible power supply systems comes in.


The functionality of a UPS extends beyond power regulation and backup, serving a number of critical functions including:

  • Voltage surge and spike protection: A UPS protects electronic equipment at times when the voltage on the line is greater than it should be.
  • Voltage sags: Some UPS units can normalize under-voltages and over-voltages, which ensures optimal operation and preserves battery life for more serious power failures.
  • Total power failure: If a line goes down or a fuse blows somewhere on the grid or in the building, a UPS can seamlessly transition to a battery reserve to ensure that all devices remain operational.
  • Intelligent communication: Advanced UPS systems can communicate with any number of connected devices, which enables automated shutdown procedures designed to preserve data integrity before total battery failure.


Types of UPS Systems

When shopping for a UPS system, there are indeed a lot of options. There's the standby UPS, the line-interactive UPS, the double-conversion UPS and a wide variety of specialized variants. Choosing the right one entails a comprehensive understanding of your particular needs paired with a modest amount of UPS system knowledge.

With that in mind, let's take a closer look at the current state of uninterruptible power supplies [source: Science Direct].


The Standby UPS

A standby UPS runs the computer off of the normal utility power until it detects a problem. At that point, it very quickly (in 5 milliseconds or less) turns on a power inverter and runs the computer off of the UPS's battery (see How Batteries Work for more information).

This type boasts features like basic surge protection and battery backup functionality. Standby UPS systems are far more common for home or small-business use because they tend to cost about half as much as more industrial alternatives.

The Line-interactive UPS

Line-interactive UPS systems are more sophisticated and are generally reserved for small- and medium-sized businesses. These essential devices are designed to deal with a wider range of power issues including voltage sags and surges, without having to switch to battery power.

The driving feature of a line-interactive UPS is its ability to correct minor power fluctuations through a multi-tap variable-voltage autotransformer. For many, they strike the perfect balance between cost and performance, and they tend to be more efficient on environments where power fluctuations happen frequently.

The Double-conversion UPS

The double-conversion UPS offers the highest level of power protection. It does this by completely isolating connected equipment from raw power sources.

Often referred to as a continuous UPS, double-conversion UPS systems continuously converts incoming power in real time, ensuring a consistent, uninterrupted power supply regardless of fluctuations and failures.

The result is an environment that's completely isolated from power anomalies, which is ideal in environments that manage highly sensitive data and critical equipment.

Popular use cases include data centers, medical facilities and telecommunications infrastructure, though they're generally recommended for the operation of all sensitive equipment, like those used in industrial settings [source: Rigby].


Eliminating Interruptions

Uninterruptible power supply systems are indispensable tools, promising untold reliability and protecting the integrity of essential electrical equipment. Whether for home use, at the office, or in large-scale data centers, UPS systems provide a critical layer of protection for sensitive electronic equipment against unpredictable power supply anomalies.

Understanding the different types of UPS systems and their functions allows individuals and businesses to make informed decisions about their power protection strategies. With the advancements in technology, UPS systems continue to evolve, offering more efficient and reliable solutions to meet the ever-changing demands of modern electronic devices.


Frequently Asked Questions

How does an online UPS work?
An online UPS is a type of uninterruptible power supply that provides backup power to a computer or electronic device by supplying power from a battery or flywheel when the input power is lost.
How long does a UPS last without power?
Typically, a UPS will last for about 15 minutes without power.