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Expert Column: UPS Basics

Important information to consider about increasingly common uninterruptible power supplies.

Expert Column: UPS Basics

Nov 1, 2007 12:00 PM,
By Peter Cook

Important information to consider about increasingly common uninterruptible power supplies.

UPS units such as the Liebert PST series are being used more frequently these days.

Taking nothing for granted, I want to start by saying that the UPS discussed in this column has nothing to do with that company with the brown trucks everybody likes to see around the holidays. In this context, “UPS” refers to uninterruptible power supply, often called “battery backup.” Due to an obvious and increasing convergence of computer technology with just about every other category of electrical or electronic device — including audio and video equipment — uninterruptible power supplies are seen more frequently these days in the required equipment list for sophisticated and mission-critical systems.

You can devise your own definition of “mission critical,” but consider anything with memory or presets or any communications device as falling into that category. Still, in spite of the growing importance of uninterruptible power supplies, there has rarely been much information presented in the trade press to describe the functions and features of a UPS — until now. Following is some useful information to consider when contemplating the purchase or specification of a battery backup device.


It is a widely held misconception that a UPS is the best form of power conditioner available. While a certain type of UPS provides clean and stable power, a broad and unqualified endorsement of uninterruptible power supplies for power conditioning is misleading. As useful as they are, the main function of a UPS is to provide brief periods of power continuation during utility power failures. Under battery power, digital files can be saved, and processors and peripheral devices can be shut down in an orderly fashion.

When considering the purchase or specification of a UPS, there are several features to consider. As with most electronic devices, the model you can buy for $99 online or at your local computer outlet won’t match the performance of a higher-grade commercial model. Then again, the $99 unit may be all you need.


“Run time” refers to the amount of time a UPS will provide power once the main utility power is interrupted. Standard battery run time for a common UPS is around 5 to 10 minutes under full load, and about twice that under half load. The battery run time during a power outage is a function of the current load placed on the battery and the size of the battery. The demand for current by the equipment being powered is expressed in amps. The amount of “juice” stored in a battery is expressed in terms of amp-hours.

If you want to know how long a given set of equipment will operate off a UPS battery, add up the current in amps that the equipment will pull and divide the amp-hours of the UPS by that number. (For instance, a half-amp load on a two amp-hour battery will run for 4 hours.) So it makes sense that the smaller the load relative to the size of the battery, the longer the run time. Over-sizing a UPS relative to the load is one way to get extended run time. For some brands, a single battery size is used for two or more sizes of uninterruptible power supplies. In this case, the run time may be much longer for the smaller unit. These figures are almost always published by the manufacturer.

Be aware that a battery will not provide full voltage through its entire rated amp-hour life. Some reduction in voltage will occur at the end of the functional operation of the battery, just as a flashlight becomes dim but still works. Some uninterruptible power supplies will provide automatic shutdown or an alarm when this voltage decline is imminent. So if you have a specific required battery run time in case of a power outage, it’s best to have a bit of amp-hour headroom on top of what you calculate.

Middle-market and top-of-the-line uninterruptible power supplies have replaceable batteries. Because even continually charged batteries have a limited life, being able to replace them is a nice feature. UPS batteries are the sealed-lead-acid type and pose no danger with normal operation, but they should be disposed of properly. Although there have been recent advances in battery technology, these components are still heavy. A 600W UPS weighs around 30lbs. A 2200W unit weighs around 80lbs. More sophisticated units allow for additional battery packs to be connected to the UPS for additional run time. Typically, these just plug into the back of the UPS.

Warning indicators for loss of main utility power and low battery power are common. A warning will also indicate when the battery is reaching the end of its useful life and needs to be replaced.


Current capacity is expressed in amperage. Most manufacturers also state the wattage capacity of their models. Because of the inefficiency of certain electrical loads, it is probably best to select a UPS size based on the wattage of the UPS and the equipment to be powered.

If only the amperage rating is available from the manufacturer, multiply it by the voltage and power factor to calculate the wattage that a UPS can produce. Select a UPS with a wattage rating a bit larger than the load. For extended battery run time, select a UPS much larger than needed for the subject load.


While most AV applications don’t require monitoring of the UPS or automated UPS control of connected equipment, most of these battery devices have a communication link to a PC. This link allows automated processor shutdown as UPS power begins to drop. In more sophisticated systems, it also allows integration of the UPS into a data network via Simple Network Management Protocol (SNMP).


With most uninterruptible power supplies, connected equipment is powered directly by the utility power until a blackout occurs. Then the UPS switches over to battery power. These are called “standby” uninterruptible power supplies. The quality of power conditioning provided by a UPS prior to a blackout varies from model to model, and it is most certainly a function of the price of the unit. If this is a concern to you, check the manufacturer’s specifications. If the manufacturer doesn’t provide power-conditioning performance specs for normal and common mode (i.e. on the ground line), don’t rely too heavily on the UPS to clean up the utility AC power. You can also check that the AC output closely simulates the sine wave of normal utility AC power when running on the battery.

Manufacturers will say that the time to switch on the battery is so brief that there is no danger of accidentally rebooting a connected processor. Having said that, when running on the battery, not all are good power conditioners — but one variety is. With an online UPS, your connected equipment never sees utility power. The UPS uses utility power only to charge the UPS battery. All connected equipment runs off battery power that has been converted to AC. This “double conversion” design provides complete isolation from incoming utility power and, therefore, makes the UPS an excellent line conditioner. An online UPS is recommended for electronic equipment sensitive to momentary voltage “dropouts” that may only last milliseconds, but that may cause system malfunctions.


Most manufacturers have floor or tabletop models, as well as rackmounted ones. As an option, the rackmounted models can be set into a stand and used in a mini-tower configuration. Because of the units’ weight, rackmounting requires special consideration and hardware.

Most units require an additional mounting shelf not included with the UPS. Because of their weight, they should always be mounted to front- and back-mounting rails. Larger, higher-current three-phase models are freestanding.


For most UPS models, installation is as simple as plugging the UPS into the wall and plugging your equipment into it. Taking advantage of the monitoring and communications features of more sophisticated models requires software loading and linking with a PC. Industrial-sized units require professional installation and startup, usually available from the manufacturer or a third-party vendor.


Keep in mind that the primary objective of a UPS is to provide temporary power during a complete blackout. As such, you need not worry about equipment that will be useless when the lights are off. In a theater setting, for instance, the PA amplifiers need not be on a UPS. However, digital signal processors or audio consoles may need battery support.

In addition, consider communications systems such as phone and pager systems, anything with memory or interconnected components, and anything that has a tendency to lock up during power glitches.

Peter Cookis vice president of Juice Goose, a manufacturer of electrical power conditioning and control technology based in Houston. He has been designing and marketing power conditioning and distribution equipment since 1988.

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