Frequently Used UPS Technologies

As we discussed in a previous post, an Uninterruptible Power Supply (UPS) is an electrical apparatus that provides emergency power to a load when the input power source fails. It does this by means of one or more attached batteries and associated electronic control circuitry. A UPS differs from an auxiliary or emergency power system in that it provides instantaneous (or nearly so) protection from input power interruptions. However, the on-battery runtime of most UPS systems is relatively short, with 5-15 minutes being typical for smaller units. Although this period seems relatively short, it is sufficient to allow time to bring an auxiliary power source on line or to properly shut down the protected equipment.

UPS units are divided into categories that are based on what type, and in some cases the number of different power related problems they address. The general categories of modern UPS systems are online, line-interactive, and standby.

The Online UPS is ideal for environments where electrical isolation is necessary or for equipment that is very sensitive to power fluctuations. Although this technology was once previously reserved for very large installations of 10 kW or more, advances in technology have permitted it to now be available as a common consumer device, usually supplying 500 watts or less.

The Online UPS is generally more expensive than other technologies but may be necessary when the power environment is “noisy,” such as in industrial settings or larger equipment loads like data centers. An Online UPS system takes incoming AC power, converts it to DC through a rectifier and feeds it to both the battery bank and an output DC to AC inverter. The UPS conditions the DC and converts it back to AC through the inverter.

In an Online UPS, the batteries are always connected to the inverter, so that no power transfer switches are necessary. When AC input power loss occurs, the input rectifier simply drops out of the circuit and power is provided by the batteries through the inverter. When power is restored, the rectifier resumes carrying most of the load and begins charging the batteries. This process means the UPS is always online since there is no delay to switch to battery.

Another significant advantage of the online UPS is its ability to provide an electrical firewall between the incoming utility AC power and sensitive electronic equipment. This type of UPS process provides a layer of insulation from power quality problems. It allows control of output voltage and frequency regardless of input voltage and frequency.

Standby UPS technology is the simplest and least expensive UPS design. In this type of UPS, the primary power source is line power directly from the utility, and the secondary power source is the battery. It is called a standby UPS because the battery and DC to AC inverter are normally not supplying power to the equipment. The battery charger is converting AC line power to DC through a rectifier to charge the battery. The battery and inverter are waiting “on standby” until they are needed.

When the incoming AC voltage goes out, or falls below a predetermined level. the UPS turns on the DC-AC inverter circuitry, which is powered from the internal bank of batteries. The UPS then mechanically switches the connected equipment on to its DC-AC inverter output. When line power is restored, the UPS switches back.

The switchover time can be as long as 25 milliseconds, depending on the amount of time it takes the Standby UPS to detect the lost utility voltage.

The line-interactive UPS uses a totally different design than any other type of standby UPS. In this type of unit the separate battery charger, DC to AC inverter, and source selection switch, have all been replaced by a combination inverter/converter. This unit both charges the battery and converts the battery DC voltage to AC for the output.

The AC line power is still the primary power source, and the battery is the secondary power source. And, when the line power is operating, the inverter/converter charges the battery. However, when the input power fails, the unit operates in reverse.

An on-line UPS also uses a double-conversion method of accepting AC input. The UPS rectifies the AC input to DC for passing through to the battery or battery strings. Then the UPS inverts the DC back to 120V/240V AC for powering the protected equipment.

The line-interactive type of UPS is able to tolerate both continuous under-voltage brownouts and overvoltage surges without consuming the limited reserve battery power. Instead, it compensates for these conditions by auto-selecting different power taps on an autotransformer. The process of changing the autotransformer tap can cause a very brief output power disruption as it briefly switches to battery before changing the selected power tap.

As you continue your study of UPS systems, you will find new technologies are becoming available, such as a Fuel Cell UPS that has been developed using hydrogen and a fuel cell as a power source. This new technology has the potential to provide long run times in a small space.

As a CCNA, you may also be called upon to repair and maintain a Rotary UPS that uses the inertia of a high-mass spinning flywheel that provides short-term ride-through in the event of power loss. The flywheel also acts as a buffer against power spikes and sags, since such short-term power events are not able to appreciably affect the rotational speed of the high-mass flywheel. It is also one of the oldest designs, predating vacuum tubes and integrated circuits.

Author: David Stahl

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  1. Jennifer Stryer Reply

    Hi David,

    I have read both of your blogs on UPS devices. Can you write an additional blog addressing how to monitor the UPS for issues, especially if you are in a situation (like me) where you do not have the possibility of two power sources. I have a small router with only one power chord. When the UPS had an issue my Call Manager did not re-register and I could not receive incoming calls.

    If you could write a blog on the best ways to monitor these devices in order to be proactive with issues, that would be great.

    Thank you.
    Jennifer Stryer
    Manager, Office Services
    Cotton Incorporated

  2. Jack Visage Reply

    An uninterruptible power supply, also uninterraptible power source, UPS or battery/flywheel backup, is an electrical apparatus that provides emergency power to a load when the input power source, typically the utility mains, fails. A UPS differs from an auxiliary or emergency power system standby generator in that it will provide instantaneous or near-instantaneous protection from input power interruptions by means of one or more attached batteries and associated electronic circuity for low power users, and or by means of diesel generators and flywheels for high power users.

  3. Alfred Reply

    I was looking into UPS for protecting my computer from shortages. What UPS do you recommend getting?
    Could you say that a Line-Interactive UPS may be the best choice for home use, since they are as much reliable as an Online UPS but less expensive (I presume)?
    What’s the difference between the price of an Online and a Line-interactive UPS?

    Great write-up on the different UPS types.
    Thanks a lot!

    AC to DC Converter

  4. Brandon Adams Reply


    A line-interactive UPS is usually less-costly than an Online UPS, and line-interactive is more commonly found in smaller, less critical applications. UPS sizing is dependent on the supported load, so it’s best to determine what devices you are supporting, what the load would be, and of much importance – what runtime are you looking to get. It’s easy to select a UPS for cheap that will accomodate your load, but if you don’t factor in a suitable runtime, you may not be able to save your work and shut down properly in the event of loss of power. The Eaton Corporation makes a full line of single phase and three phase UPS for just about every application, and they have a downloadable UPS and Power Management Fundamentals Handbook (available at: that has some great data and may be of some help to you in selecting the right UPS for your needs.