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UPS Battery Backup Buying Guide

Battery Backup for PCs, Servers and Other Electronics

This buying guide will help you:

  • Understand what a UPS system is and recognize key features
  • Learn important questions to ask before selecting a UPS system
  • Find the right size of UPS system to protect your equipment
  • Compare the different types of UPS systems available

UPS System Basics

As long as the electricity stays on, a UPS system supplies conditioned utility power to its outlets and keeps its internal battery charged. It also protects your valuable devices and data from power problems, such as power surges and abnormal voltages. If the power goes out, the UPS system provides backup power from its internal battery. This allows your equipment to stay on during a power outage, which is especially useful for devices like computers that can lose data when they turn off unexpectedly.

Note: A UPS system can also be called an uninterruptible power supply or battery backup system.

uninterruptible power supply

Between the aging electrical grid, rising power demand, severe weather, faulty wiring and disruptive devices connected to your AC line, your equipment is under constant threat from power problems. Even a brief loss of power or a momentary surge can ruin valuable electronics or corrupt irreplaceable data ranging from business records to family photos. A UPS helps prevent this from happening.

ups power supply

UPS systems are easy to use. You simply plug the UPS into the wall outlet, plug your equipment into the UPS outlets and turn on the UPS. Most UPS systems have extra features like communicating with a computer or customizing the default settings, but using the extra features is optional.

ups battery for computer

For a fraction of the value of the equipment, data and productivity it protects, a UPS system can prevent damage, data loss and downtime. It conditions incoming AC power, protects against common power problems and provides enough battery backup to outlast most outages. A huge range of models are available, ensuring there is a cost-effective solution ideal for your needs.

battery backup for computer

Blackout: Also known as a power outage or power failure, a blackout is a complete interruption of utility power. Typically caused by accidents, grid component failures, tripped breakers, severe weather or excessive power demand, blackouts may last a few seconds, a few minutes or even longer. Blackouts cause reduced productivity, lost business revenue, system crashes and data loss.

sine wave blackout

Brownout: Also known as an undervoltage, sag or dip, a brownout is a sustained deficiency in line voltage, lasting minutes to hours. Brownouts are caused by excessive power demand, either in the building or throughout the grid, and can be aggravated by poor circuit design. Localized brownouts can occur repeatedly as air conditioners and refrigerators start and stop, overloading building electrical circuits. Brownouts cause equipment failures, overheating, incremental damage, decreased stability and data loss.

sine wave brownout

Overvoltage: Also known as a swell, an overvoltage is essentially the opposite of a brownout. Instead of a voltage deficiency, an overvoltage is a voltage increase that lasts seconds to minutes – longer than a surge. An overvoltage occurs when the power provided is greater than the power accepted by connected equipment, resulting in lost data, damaged equipment, flickering lights, overheating and equipment stress.

sine wave over voltage

Surge/Spike: Surges and spikes are short-term (transient) voltage increases. Surges last three nanoseconds or more; spikes last less than three nanoseconds. They can cause catastrophic equipment damage, system freezes, data corruption and incremental damage that degrades equipment performance. They are produced by utility company load shifting, short circuits and lightning strikes. During the storm season, the incidence of surges and spikes rises with heightened thunderstorm activity and sudden power restoration after outages.

sine wave power spike

Line Noise: Line noise encompasses electromagnetic interference (EMI), radio frequency interference (RFI), harmonic distortion and waveform irregularities in line power. Though it is not always noticeable, or even detrimental, line noise can cause electronic circuit degradation, data corruption, audio/video quality problems and system malfunctions. Line noise increases with thunderstorm activity, utility power fluctuations, radio transmissions, fluorescent lighting use and cycling of air conditioning, refrigeration motors and fans that introduce interference in local electrical circuits.

sine wave line noise

Top 3 UPS Features

UPS systems perform three vital functions: providing battery backup during power outages, regulating abnormal voltages and protecting equipment from surges and line noise.

As long as electricity stays on, the UPS system provides utility power to the devices connected to its outlets, keeps its internal battery charged and protects equipment from power problems. During an outage, the UPS system keeps connected devices functioning by supplying electricity from its internal battery.

The battery backup runtime of the UPS is the estimated time it will keep connected equipment powered during an outage, without an opportunity to recharge its batteries. Runtime varies depending on the size of the UPS battery and the wattage required by the connected equipment, as well as factors like efficiency, room temperature and battery age. Each UPS product information page at provides access to estimated runtimes for the entire range of wattages the UPS can support.

In the event of an extended blackout that exceeds the backup runtime of the UPS, battery backup provides an opportunity to shut down computers properly and prevent data loss. Automatic shutdown is also available for unattended computers. For applications where shutting down is not an option, many network and mission-critical UPS systems support increasing battery capacity to extend runtime from minutes to hours.

Network and mission-critical applications often include standby generators as part of the facility's backup system, so UPS systems may only require sufficient runtime to support the equipment load until generators start up, stabilize and take over. While generators are operating, UPS systems continue to condition power before it reaches connected equipment.

ups power backup desktop ups server battery backup

All Eaton UPS systems protect connected equipment against abnormally low voltages (known as brownouts, sags or undervoltages) and high voltages (known as swells or overvoltages) that can cause malfunctions or damage hardware. However, the three main types of UPS designs (known as UPS topologies) accomplish this in different ways.

ups voltage

Standby UPS systems switch to battery when they detect abnormal line voltages. This protects equipment, but only as long as the UPS system's battery backup runtime lasts. In locations with frequent voltage problems, whether from substandard utility power or adverse conditions at the installation site, frequent switching to battery and associated recharging cycles can also shorten the lifespan of the battery.

standby battery backup

Line-interactive UPS systems include a feature called automatic voltage regulation (AVR) to correct abnormal voltages without switching to battery. The UPS system detects when input voltage is too low or too high and automatically boosts or reduces the voltage by a set percentage before passing it to connected equipment. Line-interactive UPS systems typically regulate output within 10-15% of the nominal voltage.

Note: Some low-cost line-interactive UPS models only correct low voltages, which are more commonly the source of problems. The specifications of the UPS system will indicate which voltages are regulated by AVR.

network battery backup

On-line UPS systems use a technology called double conversion to provide the strongest protection against abnormal input voltage and the most precise regulation of output voltage. This makes them the best choice for mission-critical equipment, sites with challenging power conditions and devices that are especially sensitive to power quality. The UPS continuously converts AC input power to DC power, then converts the DC power to high-quality AC output power for connected equipment. On-line UPS systems can accept the widest range of input voltages and typically regulate output within 2-3% of the nominal voltage.

Note: The high-speed cooling fans in on-line UPS systems may be too loud for quiet home and office environments. Contact us before selecting an on-line UPS for a desktop application.

battery backup for server

Built-in surge suppression and noise filtering shields equipment from destructive transient voltages and disruptive electromagnetic and radio frequency (EMI/RFI) line noise. The UPS provides the functionality of a surge protector without requiring a separate protective device. Some UPS systems also include data line protection to protect equipment from surges on phone, Ethernet or coaxial lines.

battery backup surge protector

UPS Systems by Use

Our online UPS finder allows you to select UPS systems by recommended application. This gives you a head start when narrowing choices from hundreds of possibilities, but you should also review the rest of this guide before making your final selection. Do not hesitate to contact us if you have any questions.

best ups for pc
Computer/Peripheral UPS Systems
  • Protect desktop computers, peripherals and other home and office electronics
  • Upgrade to Home/Office Desktop UPS Systems if you want more protection, including automatic voltage regulation (AVR)
best ups for small business
Home/Office Desktop UPS Systems
  • Protect desktop computers and other electronics, such as monitors, wireless routers, modems, smart speakers, POS systems and kiosks
  • Consider Computer/Peripheral UPS Systems for lower-cost options without automatic voltage regulation (AVR)
best ups for audio equipment
Audio/Video UPS Systems
  • Protect televisions, DVRs, game consoles, media players, disc players, digital signage and related audio/video equipment
  • Choose a standby model to reduce cost; choose a line-interactive model to increase protection
best ups for servers
High-End Desktop / Small Network UPS Systems
  • Protect high-end computers, workstations, workgroup servers and network closets
best ups rack mount
Large Network UPS Systems
  • Protect servers and network hardware in data centers and server rooms
best ups for server room
Mission-Critical UPS Systems
  • Provide maximum protection against downtime for critical servers and network hardware
ups voltage regulation and isolation
Hospital/Medical UPS Systems
  • Protect equipment in healthcare settings, including models for patient care areas

How to Choose a UPS: 7 Key Questions

UPS types (or topologies) include standby, line-interactive and on-line. They are explained in more detail above in Top 3 UPS Features and compared below in the UPS Comparison Chart.

If you are selecting a UPS for a home or office application, you will likely choose between standby and line-interactive UPS systems. Standby UPS systems typically cost less and have some of the smallest space requirements. Even if cost is your primary concern, however, always consider a line-interactive UPS. The price difference is negligible in many cases. If you want to protect valuable and sensitive computers and electronics, especially in locations with unstable utility power or frequent storms, choose a line-interactive UPS whenever possible.

If you are selecting a UPS for a network application outside your home, such as protecting servers and network hardware for a business, you will likely choose between line-interactive and on-line UPS systems. Generally speaking, line-interactive UPS systems have lower acquisition costs and on-line UPS systems provide significantly more protection. The question comes down to whether the extra protection is worth it to you, and that usually depends on your application and tolerance for disruption and downtime. How much does it cost to replace damaged equipment? How much does it cost in revenue and aggravation if your network is down for a while?

For most types of equipment, line-interactive UPS systems provide excellent, cost-effective protection. For critical equipment essential to productivity, devices sensitive to power quality and locations with difficult power conditions, on-line UPS systems are a better choice. They provide complete isolation from most power problems, the strongest protection against costly downtime and the best compatibility with sensitive equipment.

Key Point: If budgetary considerations require you to use a line-interactive UPS where you would prefer an on-line UPS, make sure it provides pure sine wave output in battery mode instead of pulse width modulated (PWM) sine wave output.

how to choose a battery backup how to choose ups for pc how to choose a ups how to know which ups to buy

A UPS system's output capacity is the maximum power it can supply in battery mode to support connected equipment. It is typically measured in volt-amperes (VA). The capacity of the UPS must be at least as large as the total power required by the connected equipment or it will experience an overload and be unable to support the equipment during an outage.

To estimate power capacity requirements, add up the wattage of the devices you plan to connect to the UPS. Refer to the nameplate or manufacturer's documentation for each device to find its wattage. If output is listed in amps, multiply by the nominal AC voltage to estimate wattage.

UPS systems designed for home and office use are in the 300 to 1,500 VA range, allowing them to plug into standard household outlets. UPS systems designed for data centers may be much larger. This is also a function of the type and quantity of equipment that is likely to be connected: an energy-efficient desktop computer may only require 60 watts, while a data center rack packed with servers will require thousands of watts.

Key Points: To leave a margin for error and fluctuating power demands during operation, select a UPS with an output capacity (VA) rating 20% higher than the estimated power requirements of your connected equipment. Also consider whether you want to leave room for future expansion. We recommend that you confirm your estimate with our technical support team, especially for more complex situations and applications requiring 5,000 VA or more.

how big of a ups do i need how much ups do i need how big of a battery backup do i need

The AC input voltage (and input plug type) of the UPS should match the voltage (and wall outlet type) of the installation site. For homes and offices in North America, the input (and output) voltage is typically 120V and the input plug type is NEMA 5-15P, which is the familiar grounded power plug used for most household applications. For data center applications in North America, there is a range of plug types to choose from, including 120V (5-15P, 5-20P, L5-30P) and 208/240V (L6-20P, L6-30P) plugs that support a range of power capacities. Large capacities may require a hardwire input connection installed by an electrician, rather than a detachable plug.

Key Points: The input voltage (and plug) should match the voltage of a circuit available (or installable) at the site. The output voltage should match the requirements of the connected equipment. For home and office applications in North America, nominal AC input and output voltages are typically 120V.

how much va ups do i need

The runtime of the UPS system is an estimate of how long its battery can support connected equipment during a power outage. The runtime varies with the wattage of the equipment connected to the UPS system's outlets. The higher the wattage, the shorter the runtime. The UPS system's product detail page will list estimated runtimes at specified wattage levels. If you need additional runtime, choose a UPS system with a higher load capacity and/or expandable runtime.

Key Points: At 80 percent of its maximum capacity, a UPS system will provide several minutes of runtime during a power outage. That's long enough to outlast most outages and gives you time to shut down equipment (either manually or automatically) during a longer outage.

calculate how long ups will last

When operating from battery during an outage, a UPS system generates the waveform of its AC output. Many UPS systems generate an approximation of the sine wave power you receive from your electrical utility instead of a pure sine wave. Although this approximation is compatible with most equipment, pure sine wave power is required by some devices – such as iMacs and other computers with active PFC power supplies – and prevents others from overheating, malfunctioning or failing prematurely. UPS systems that provide pure sine wave power from battery may also offer superior compatibility with sensitive equipment, such as network hardware and high-end audio/video components. All on-line UPS systems and many line-interactive UPS systems supply sine wave output from battery.

pure sine wave

This seems straightforward – simply count the number of devices that you plan to plug into the UPS system. But you should also consider that AC adapters may cover more than one outlet. Some UPS systems include outlets with extra space around them to accommodate bulky AC adapters without blocking adjacent outlets.

If you are connecting servers or network equipment, each device may have more than one internal power supply and more than one power cord.

Key Points: UPS systems designed to be mounted in a data center rack typically work in conjunction with one or more separate power distribution units (PDUs). Each PDU plugs into an outlet on the UPS, and IT equipment plugs into outlets on the PDUs. In this case, the UPS only needs enough outlets to plug in the PDUs.

how to choose a battery backup for computer

The form factor of the UPS refers to its shape and position.

how to choose ups for desktop

A desktop UPS (also known as low-profile or flat-pack) is shorter in height than width and typically has outlets on top, similar to a power strip. It might be on a desk, but it could also be on the floor or mounted to a vertical surface. Sometimes people prefer to set a desktop UPS on end, so the outlets face forward.

how to buy ups for computer

A tower UPS is shorter in width than height (or roughly the same) and typically has outlets in back.

how to choose ups for server

A rack-mount UPS is specially designed to be mounted inside a standard rack enclosure or open frame rack. The UPS system's rack height specification indicates how many vertical rack spaces it will require. (One rack unit – 1U – is 1.75 inches high. A full-size rack is 42U, and a rack-mount UPS may require from 1U to 14U.)

Some UPS systems support more than one form factor. For example, most rack-mount UPS systems can also rest on the floor in a tower orientation. Optional tower stands provide extra stability for this application.

Key Points: Also check the specifications for the UPS dimensions and the length of the input power cord. The UPS has to fit in the final installation location – without blocking cooling fans – and the cord has to reach the nearest wall outlet comfortably. The weight of the UPS and any external batteries may also be important, especially for heavy data center models that can overload racks – or even standard floors.

ups rack mount

More UPS Features Explained

Communication Ports / Remote Management

Most UPS systems have built-in USB and/or serial communication ports that enable power management and automatic unattended shutdown when the UPS is connected to a computer. Most UPS systems designed for network applications also have a slot for an optional network management card. (Some UPS systems include a pre-installed card.)

The network management card enables comprehensive management via SNMP, Web, SSH or telnet. It has an Ethernet port, allowing you to access advanced monitoring, control and notification features over the network without directly connecting the UPS to a computer. The card also supports connecting one or more optional sensors for remote temperature, humidity and contact closure monitoring.

ups features remote management
Cooling Fans

Larger UPS systems typically have built-in cooling fans, which may affect the noise level of your environment during operation. The high-speed cooling fans in on-line UPS systems may be too loud for quiet home and office settings. Contact us before selecting an on-line UPS for a desktop application.

ups features cooling
Data Line Surge Protection

Data and audio/video lines connected to your equipment represent an unlocked "back door" that allows surges to enter and damage or destroy sensitive electronic components. Select UPS models have RJ11 jacks, RJ45 jacks or coaxial connectors to protect equipment against surges on connected phone, Ethernet network or cable lines.

ups features surge
ENERGY STAR Certification

UPS models with the ENERGY STAR logo are third-party certified to help you save money, save energy and protect the environment by meeting strict efficiency guidelines set by the U.S. Environmental Protection Agency.

ups features efficiency
Isolation Transformer

A UPS with a built-in isolation transformer provides immunity from EMI/RFI line noise and ground loops, reducing data errors and audio/video distortion for point-of-sale systems, computers, network equipment, audio equipment and home theater systems. This feature is especially useful for applications that require long data cable runs.

ups features isolation
LCD Control Panel

A control panel with an LCD screen allows you to change UPS settings or issue commands without using a computer. It can also display helpful information like input voltage or battery capacity. (Some UPS systems with multiple LEDs can also indicate this information, though less precisely.)

ups features control panel
Lithium Battery

Select UPS systems have a lithium-iron-phosphate (LiFePO4) internal battery, a special type of lithium-ion battery optimized for safety and durability in high-power applications. Compared to the lead-acid batteries found in traditional UPS systems, lithium batteries offer longer lifespan, more charge/discharge cycles, faster recharging and lighter weight. Although they are initially more expensive than lead-acid batteries, they require replacement less often. Lead-acid batteries typically last three to five years, while lithium batteries may last ten years or more. Lithium batteries are especially attractive for applications where accessing the UPS to replace worn-out batteries is difficult and/or costly.

ups features lithium
Muted Battery Alarm

UPS models with a muted battery alarm remain silent by default until the battery is low, preventing nuisance beeping caused by brief power outages.

ups features alarm
Replaceable Battery

Most UPS systems have a removable battery panel. This allows you to replace the internal battery after it wears out, rather than replacing the entire UPS. (Most UPS batteries last three to five years under normal conditions.) Eaton offers a complete line of replacement batteries.

ups features battery replacement
Economy Mode

On-line UPS systems support user-selectable economy mode (also called ECO mode) operation to save energy and reduce operating costs. While input power quality is good, the UPS operates with maximum efficiency. If input power quality worsens, the UPS switches to double conversion to provide maximum protection.

ups features economy
Energy-Saving Outlets

Every AC adapter and glowing LED represents a potential "phantom load," which is electricity wasted by an idle device, even when it appears to be turned off. Eco UPS systems help eliminate phantom loads.

When the energy-saving function is enabled, the UPS detects when the designated "master" device, such as a computer or television, turns off or enters standby mode. Then the UPS automatically turns off the energy-saving outlets to prevent peripheral devices connected to those outlets from wasting electricity. Other outlets stay on to support devices that operate continuously, such as wireless routers and DVRs. When the master device turns on again, the UPS automatically restores power to the energy-saving outlets, and the entire system is ready to use.

ups features energy saving
Surge-Only Outlets

Select models designed for home and office applications have two groups of outlets. Standard UPS outlets provide battery backup and surge/noise protection for essential devices like computers and keep them powered during outages. Surge-only outlets protect nonessential devices like printers from damaging surges while preserving load capacity and battery backup runtime for the devices connected to the standard UPS outlets.

ups features surge protection
USB Charging Ports

USB charging ports on select models provide convenient charging of mobile devices, including smartphones, tablets, portable media players, GPS devices, fitness trackers, power banks, digital cameras and handheld games.

usb features charging
Automatic Internal Bypass

On-line UPS systems include an internal bypass that automatically passes through utility power to keep equipment powered in the event of a fault or overload. This helps eliminate downtime.

ups features bypass
Battery-Independent Restart

Battery-independent restart ensures automatic UPS startup without user interaction after lengthy power outages that completely drain the UPS batteries, even if the batteries require replacement. This keeps downtime to a minimum for critical applications and reduces the need for emergency site visits.

ups features automatic restart
Emergency Power Off

UPS systems designed for data center applications typically include provisions for connecting to your facility's Emergency Power Off (EPO) circuit. During an emergency, such as a fire, activating the EPO switch will immediately de-energize all the equipment connected to the EPO circuit.

ups features power off
Expandable Runtime

Many UPS systems designed for network applications can connect to hot-swappable external battery packs to expand runtime to match any requirement, including the most demanding extended-run applications.

ups features runtime
High Power Factor

Some network/server UPS systems have a high power factor that makes more efficient use of your facility's power infrastructure and available space. A high power factor allows you to connect more equipment to each UPS system and circuit. While a 10,000 VA UPS with a 0.8 power factor will support an 8,000-watt load, a 10,000 VA UPS with a 0.9 power factor will support a 9,000-watt load, an increase of 1,000 watts without a corresponding increase in circuit requirements.

ups features power factor
Hot-Swap / Manual Bypass

UPS systems are crucial to maintaining system availability, but the wrong UPS system can actually cause downtime. Traditional UPS systems typically require you to power down and disconnect your equipment during a UPS repair. Before you can restore productivity, you will need a technician to install the new UPS system. You will also need to reconnect, power up and test your equipment.

Eaton's hot-swappable UPS systems have one or more user-installable, hot-swappable power modules and a detachable PDU with a manual bypass switch. After you activate the bypass switch, you can completely remove and replace the power module while the detachable PDU keeps your critical systems powered. This feature can eliminate up to 60 minutes of downtime during a single repair.

ups features hot swap
Industrial-Strength Overload Capacity

Eaton's on-line UPS systems support higher inrush currents at startup and fluctuating power demands during use without shutting down production lines. They handle large overloads for brief periods and transfer to bypass dynamically to sustain operation during higher and longer overloads for extended periods. This feature is especially useful in factory manufacturing environments with heavy inductive motor loads.

ups features overload
Isolation from Power Problems

The double conversion process allows on-line UPS systems to isolate connected equipment from most power problems on the AC line. This includes problems that can be difficult to solve with other UPS types, like harmonic distortion, electrical impulses and frequency variations. They also provide superior protection against blackouts, brownouts, overvoltages, surges and line noise.

ups features double conversion
Load Banks

Select UPS systems designed for network applications support switching groups of outlets off or on independently of other UPS outlets. These "load banks" allow you to reboot select equipment remotely without power cycling the entire UPS. They also support automated load shedding to prioritize the runtime of critical equipment during extended outages.

ups features load bank
Single-Phase vs. 3-Phase Power

Single-phase is the type of AC utility power found in the typical home or office. 3-phase power circuits simply provide higher capacity than single-phase power circuits, much like high-voltage circuits provide higher capacity than low-voltage circuits. If your application requires a UPS smaller than 10,000 VA (10 kVA), it will require single-phase input power. If your application requires a UPS larger than 20 kVA, it will require 3-phase input power. If your application falls in-between, single-phase and 3-phase input power are both possibilities. Because sizing a large UPS can be complex, we recommend that you contact us for assistance with applications requiring 5,000 VA or more.

ups features single phase
Voltage- and Frequency-Independent (VFI) Operation

VFI operation is another name for the double conversion operation of an on-line UPS system. It highlights the on-line UPS system's ability to isolate connected equipment from voltage and frequency problems in the AC supply circuit by converting incoming AC power to DC power, then converting the DC power to ideal AC output.

ups features vfi operation
Zero Transfer Time to Battery

Since an on-line UPS system is already converting from DC to AC power as part of the double conversion process, the transfer time between power failure detection and power delivery to your equipment during an outage is instantaneous (zero). While the transfer time of a line-interactive UPS system designed for network applications is extremely fast (two to four milliseconds), even this short delay may cause sensitive equipment to malfunction or shut down.

ups features transfer time

UPS Comparison Chart

Type/Topology Standby Line-Interactive for Desktops Line-Interactive for Networks On-Line (Single-Phase) On-Line (3-Phase)
Typical Use PCs, home electronics PCs, electronics, home networks High-end PCs, servers, network equipment Critical servers and network equipment Critical servers and network equipment
VA Range 300 - 1,400 300 - 1,500 500 - 5,000 750 - 20,000 10,000 - 400,000
Battery Backup
Surge Protection
Voltage Regulation without Using Battery
Sine Wave Output from Battery
Double Conversion and Zero Transfer Time
Auto Bypass
Manual Bypass
USB and/or Serial Ports
Remote Management Card (Slot or Pre-Installed)
  • Most or all models support this feature.
  • Some models support this feature.
  • Only a few models (or one) support this feature.

UPS & Battery Recycling

Did you know that lead-acid batteries are recycled at a higher rate than any other consumer product? According to Battery Council International, they have a 97% recycling rate, compared to 55% for aluminum cans and 45% for newspapers.

Recycle your old UPS systems and batteries to support a cleaner, more sustainable environment.

Learn How

ups features battery recycle

Why Buy from Eaton?

We know you have many brands to choose from. On the surface, they may all seem alike. It's what you don't see that makes the difference. With Eaton, you get solid engineering, proven reliability and exceptional customer service. All our products undergo rigorous quality control before they are offered for sale, and independent testing agencies verify our products meet or exceed the latest safety and performance standards. Our commitment to quality allows us to back our products with industry-leading warranties and responsive customer service. It's the Eaton difference.

Glossary of UPS Terms


Alternating Current (AC)
A type of current that alternates from positive to negative at regular intervals. AC is the standard type of current used in electrical distribution systems by utility power companies due to the ease that it travels through cabling. Electrical wall sockets in nearly all structures served with utility power provide AC power.
Ampere (Amp) (A)
The unit of measure for electrical current.
Apparent Power
The load power as expressed in VA or KVA (e.g. 3,000 VA UPS, 20 kVA UPS System). This value is usually greater than real power. See Real Power.


A group of cells connected in such a way that more current and/or voltage is delivered than from one cell. See Direct Current.
An AC power failure lasting anywhere from a few cycles to several hours or even days in duration. Also known as a power failure. Synonymous with Outage.
Short for circuit breaker. See Circuit Breaker.
Common term for undervoltage, taken from the coloration of filament style light bulbs during undervoltage conditions. See Undervoltage.


Charge Voltage
The voltage that must be applied to storage batteries to maintain their maximum charge.
Circuit Breaker
A resettable device that responds to a preset level of excess flow by opening the circuit, thereby preventing damage to circuit elements.
Clamping Voltage
The maximum voltage allowed on an electrical circuit due to the operation of surge-protective devices. When line voltage exceeds the clamp voltage of the suppression components, the signal is diverted to the ground. The clamping voltage of a product is important because it indicates when the surge protector "clamps" a surge- or noise-related condition. North American AC surge suppression products typically have a clamping voltage around 140V. (International products clamp around 300V.)
Common Mode Voltage
The voltage present when measuring between neutral to ground.
The flow of electricity in a circuit as expressed in amperes (amps). See Amperes.


See Direct Current.
Decibel (dB)
The standard unit of expressing the loss or gain of electrical power in a circuit.
A short duration brownout condition, usually in response to inductive loads starting and stopping. See Brownout.
Direct Current (DC)
A type of electricity where current flows in one direction, without reversal such as from a battery.
Any deviation from the normal sine wave for an AC quantity. Alternating waveforms with a square or rectangular waveshape carry some amount of distortion. Typically, a good AC supply waveform will carry 5% THD (total harmonic distortion) content or less. See Harmonics, Total Harmonic Distortion.
Double-Conversion UPS
See On-Line Double-Conversion UPS.
Dropout Voltage
The AC voltage at which a device fails to operate properly and/or safely. Most 120V computer systems will shut down, reboot, reset or lose data when line voltage falls below 100V or so.


The ratio of output energy to input energy for a device. Often refers to the amount of energy lost in the form of heat during DC to AC operation.
Electrical Interference (EMI, RFI, EMP, ESD)
These are acronyms for four common types of electrical interference: electromagnetic interference (EMI), radio frequency interference (RFI), electromagnetic pulse (EMP) and electrostatic discharge (ESD). All four are unwanted signals common in noisy electrical environments.
Electromagnetic Interference. See Electrical Interference.
Electromagnetic pulse. See Electrical Interference.
Energy Absorption
The amount of electrical energy absorbed by a device measured in joules (J).
Electrostatic discharge. See Electrical Interference.


Energy that is fed from the output of a circuit back to its input.
An electronic device that blocks the passage of certain frequencies while allowing other frequencies to pass.
Common formulas necessary to properly size UPS products:
Amps = Volt Amps / Nominal Voltage
Volt Amps = Nominal Volts x Amps
Watts = Volt Amps x Power Factor
The number of cycles (oscillation positive and negative) completed in one second. In North America, utility power completes 60 cycles per second or 60 Hz.
A circuit protection device that automatically self-destructs when the current passing through exceeds the rated value of the fuse. This prevents the excessive current from damaging connected equipment.


The combination of a motor driving an electrical generator. Gasoline, propane or diesel generator systems are used in conjunction with electronic UPS systems for long-term operation during extended power outages. They are frequently employed in healthcare, emergency and other highly critical applications. Generators may require several minutes to start up before being able to provide stable, reliable output. Certain types of less-sophisticated generators may have a problem with output frequency regulation.
A conducting connection, whether intentional or accidental, between an electrical circuit and the earth, or to some conducting body that serves in place of earth.
Ground Loop
The condition of having two or more ground references in a common system. When two or more grounds have a potential difference between them, current can flow. This flow of current is a new circuit or loop which can interfere with the normal operation of the system.


An AC power connection requiring installation by a qualified electrician who connects individual wires according to NEC (National Electrical Code) standards. Some large UPS systems require hardwire input and output connections instead of plugs and outlets. Hardwire input is more common than hardwire output.
A frequency that is a multiple of the fundamental frequency. For example, 120 Hz is the second harmonic of 60 Hz, 180 Hz is the third harmonic, etc.
Harmonic Distortion
Excessive harmonic content that distorts the normal sinusoidal waveform.
Hertz (Hz)
Refers to the frequency of alternating cycles in an AC waveform per second. In North America, utility power is provided at 60 Hz. In Europe and much of the rest of the world, utility power is provided at 50 Hz.
Hot Swappable Battery
Refers to the feature that allows the battery of a UPS to be changed (due to age or defect), without taking the unit (and its attached load) out of service.
Abbreviation for hertz. See Hertz.


Measured in ohms, impedance is the total opposition to current flow in a circuit where alternating current is flowing.
Inductive Load
An electrical load with a current waveform lagging the voltage waveform, thus having a lagging power factor. Some inductive loads, such as electric motors, have a large startup current requirement. See Inrush Current, Non-Linear Load.
Inrush Current
The initial surge of current into a load before it reaches its normal operating condition. Certain types of loads, such as motors, compressors, air conditioners, power tools and other inductive loads, require two to five times more energy at startup than they do continuously. Motorized loads may start and stop many times during operation. See Inductive Load.
The subassembly of a UPS that converts DC power to AC power.
Isolated Filter Bank
A feature of the Isobar surge protector that prevents noise created by equipment plugged into one filter bank from interfering with equipment plugged into a separate filter bank.
Isolation Transformer
A transformer used to reduce or eliminate noise and create the equivalent of a dedicated or isolated ground circuit. Standalone isolation transformers serve the function of removing common mode noise. See Common Mode Voltage.


A measure of electrical energy, often used to rate a surge protector's ability to absorb energy.


Abbreviation for kilo-volt-amperes, a unit of measure of apparent power. (1 kVA = 1,000 VA.) See VA.
Abbreviation for kilowatt, a unit of measure of real power. (1 kW = 1,000 watts.) See Watt.


Line Interactive UPS
Line-interactive systems switch to battery power when blackouts occur, but when brownouts or overvoltages occur, a tap-switching voltage regulation circuit activates to maintain usable power at the output continuously, without using battery power. The main benefit is that connected equipment can run straight through extended brownouts or overvoltages without draining the battery. It also prevents frequent voltage problems from reducing the battery's lifespan. Line-interactive UPS systems are widely considered mid-level products between basic standby UPS systems and higher-end on-line UPS systems. See On-Line Double-Conversion UPS, Standby UPS, Tap Switching.
The equipment (such as a server) that uses power supplied by the source (such as a UPS).


One of the conductors of a 3-phase "wye" system.
Unwanted electrical signals that produce undesirable effects in the circuits of the control systems in which they occur. See Electrical Interference.
Nominal Voltage
A nominal value assigned to a circuit or system for the purpose of conveniently designating its voltage classes (120V AC, 208/240V AC, 12V DC, etc.). For example, a nominal 120V wall outlet will rarely measure exactly 120V. The nominal (or named) voltage of 120V refers to a range of usable voltages located near 120V.
Non-Linear Load
Electrical load that draws current discontinuously or whose impedance varies throughout the cycle of the input AC voltage waveform. It typically involves electronic devices that pull a great deal of startup current. Some examples are motors, heating elements, air conditioners, power tools and compressors. See Inductive Load, Inrush Current.
Normal Mode Voltage
A voltage that appears between or among active circuit conductors. A 120V wall outlet should yield full nominal voltage between hot and neutral line connections. See Common Mode Voltage.


Off-Line UPS
Another term for Standby UPS. See Standby UPS.
A unit of electrical resistance between two points in a conductor, such as a wire.
On-Line Double-Conversion UPS
A high-end UPS design where output power is completely regenerated and passed to connected equipment with zero transfer time between line and battery power. Incoming AC power is converted to DC and then converted back to AC by a continuous-duty inverter system. The dual conversion process completely regenerates the power flowing from an on-line UPS, completely removing all surges, spikes, noise and most other irregularities, to provide pure, frequency-regulated sine wave output at all times. On-line, double-conversion UPS systems are widely considered the best possible type of UPS available. See Standby UPS, Line-Interactive UPS.
Orderly Shutdown
Sequential shutdown procedure used on a computer system to prevent damage to the system or unwanted actions by any of the system's units. For example, a computer typically requires an orderly shutdown to preserve data integrity. Also known as a graceful shutdown.
A complete loss of voltage resulting from a localized utility failure. Also known as a power failure. Synonymous with Blackout.
When used to describe a specific type of extended variation, overvoltage refers to a voltage having a value of at least 10% above the nominal voltage for any lasting period. This occurrence may last a few seconds, several hours or become a continuous condition, depending on the site and prevailing conditions. An overvoltage differs from a surge because it is a condition of less severe voltage levels that lasts a minimum of several cycles.


Peak Voltage
A measurement of an AC waveform of the highest peak-to-peak voltage present on the waveform. A properly synthesized 120V nominal AC waveform will have a peak voltage (also known as "peak-to-peak" voltage) of approximately 170V.
Point of Use
As applied to power protection, point-of-use protectors are placed near equipment, as opposed to placement at branch circuits or utility power entrances. Plug-in surge protectors, voltage regulators and UPS systems provide point-of-use protection since the protected equipment plugs directly into the power protection device. The use of remote panel-mount whole circuit protectors does not take the place of point-of-use protection, as it's estimated that more than 60% of surges present in a typical computer circuit are generated from equipment interaction occurring downstream of major power distribution panels.
Power Factor (True)
The ratio of active power in watts (W) to apparent power in volt-amps (VA).
Power Factor = W ÷ VA
W = Power Factor x VA
Pseudo Sine Wave
See PWM Sine Wave.
Pulse Width Modulation
The process of modulating a pulse train by varying the pulse width proportionately to the modulating signal. Typically refers to a Pulse Width Modulated (PWM) UPS output waveform in battery mode. Usually implies less than a true sine wave output with THD levels around 20%. See PWM Sine Wave.
Pure Sine Wave
See Sine Wave.
PWM Sine Wave
Considered a mid-level waveform, a PWM sine wave is suitable for all but the most sensitive of computing applications. Rather than the smooth arc typically associated with a sine waveform, a PWM sine wave offers several rectangular steps to help mimic the energy supply of a sine wave. See Pulse Width Modulation, Sine Wave.


Real Power
The load power as expressed in watts. This value is usually lower than apparent power, expressed in volt-amps. See Apparent Power.
Duplicating devices to the extent that if one were to fail there would be an identical unit to replace the failed unit. Often employed in mission critical networking systems as "mirrored" or "redundant" servers where both machines are performing identical tasks. If one of these servers fails, the application will keep functioning.
Response Time
The time it takes for a suppressor to sense a surge or spike and react to it.
Radio frequency interference. See Electrical Interference.
Acronym for Root-Mean-Square, a mathematical formula used to calculate the effective values of time-variant waveforms. 120V nominal voltage levels are RMS levels. See Peak Voltages.
Rolling Blackouts / Rolling Brownouts
A condition where power utilities are forced to create undervoltages and blackouts intentionally within their service area to create spare capacity so the entire system is not at risk for power failure. When power usage for a given area or community exceeds 95% of capacity, utilities may purposely create brownouts to increase available capacity. If the condition worsens and further brownouts will not liberate enough energy, the next step is to create blackouts. See Blackout, Undervoltage.


Common term for Undervoltage.
Sine Wave
Describes an ideal AC waveform with a smooth, arcing waveform. All products using AC power are designed for use with sine wave output, also known as pure sine wave. Many high-end line-interactive and on-line UPS products provide sine wave output at all times, even when operating from battery. See Square Wave, PWM Sine Wave.
Single-Phase Power
Single-phase is the type of AC utility power found in the typical home or office. It has a single AC waveform present. Long-range utility transmission power lines carry 3-phase power. High-capacity applications (above 10 kVA) may require 3-phase input power, with 3-phase and/or single-phase output. See Three-Phase (3-Phase) Power.
SNMP (Simple Network Management Protocol)
A widely used network monitoring and control protocol. Data is passed from SNMP agents, which are hardware and/or software processes reporting activity in each managed network device, to the workstation console used to oversee the network. The agents return information contained in a MIB (management information base), which is a data structure that defines what data is obtainable from the device and what actions/functions can be controlled.
A short-term voltage increase that lasts less than three nanoseconds.
Acronym for standby power supply. See Standby UPS.
Square Wave
Describes an alternate AC waveform that is considered by many to be the least desirable for computing applications. Rather than the smooth arc typically associated with a sine waveform, a square waveform is rectangular and may complicate the operation of sensitive electronics that are used continuously or for long duration. Before 1990, many standby UPS systems provided square wave output in battery mode. Some low-cost power inverters still offer square wave output. See PWM Sine Wave, Sine Wave.
Standby UPS
A UPS that passes utility power to the output when conditions are stable, but switches to battery power when line voltage reaches a set low or high point. Standby UPS systems are often used to protect home computers and peripherals. Less frequently known as SPS (standby power supply) or off-line UPS. See Line-Interactive UPS, On-Line UPS.
A short-term voltage increase that lasts three nanoseconds or longer.
See Overvoltage.


Tap Switching
A procedure where the coil within an autotransformer is changed to maintain the output voltage at a level "independent" from the source level. Line conditioners and line-interactive UPS systems use tap-switching voltage regulation to maintain acceptable output voltage levels to connected equipment during brownouts and overvoltages.
A terminal emulation protocol commonly used on the Internet and IP-based networks. It allows a user at a terminal or computer to log onto a remote device and issue commands through a command-line interface (CLI). Most devices that allow telnet access require users to have an established account and password.
See Total Harmonic Distortion (THD).
Three-Phase Power (3-Phase)
3-phase power is AC power supplied on three separate waveforms with a phase difference of 120 degrees. Long-range utility transmission power lines carry 3-phase power, and 3-phase power is generally delivered to the service entrance of a large building rather than single-phase power. 3-phase power circuits provide higher capacity than single-phase power circuits, much like high-voltage circuits provide higher capacity than low-voltage circuits. Single-phase UPS systems max out around 10 kVA per power module. Larger UPS systems require 3-phase input, though they typically supply single-phase power to IT equipment. (The three phases can be split into three single-phase outputs.) See Single-Phase Power.
Total Harmonic Distortion (THD)
The ratio of the RMS (root-mean-square) of the harmonic content to the RMS value of the fundamental quantity, expressed as a percent of the fundamental. Typically, a supply sine wave is considered acceptable when THD levels are 5% or less. See Distortion, Harmonic, Harmonic Distortion.
Transfer Time
The time it takes to switch from AC line power to battery power. Sensitive electronic devices may malfunction or shut down when subjected to a long transfer time.
A device used to increase or decrease voltages in AC power applications.
Acronym for transient voltage surge suppressor. This is another term for surge protector, taken from the UL designation for this type of device. See Surge.


Undervoltage refers to a measured voltage having a value at least 10% below the nominal voltage for a period greater than one minute. Also known as sags or brownouts, undervoltages frequently occur during air conditioning season due to peak power demands. They may last minutes, hours, days or longer. See Rolling Blackouts / Rolling Brownouts.
Uninterruptible Power Supply (UPS) System
A UPS system, also known as a battery backup system, provides continuous, acceptable power to connected equipment loads no matter what is (or is not) coming in on the commercial utility's power lines (within limits). UPS systems provide battery backup power when utility power fails. See Standby UPS, Line Interactive UPS, On-Line Double Conversion UPS.
User-Replaceable Battery
A feature that allows the battery of a UPS to be removed and replaced without requiring the services of a technical specialist.


Abbreviation for volt-amperes or volt-amps, which is the unit of measurement of apparent power. Most UPS systems are rated in volt-amps. Actual wattage ranges from 50% to 100% of this figure, depending on the UPS system's power factor. See Ampere, Apparent Power, Formulas, Power Factor, Real Power, Watt.
Volt (V)
A unit of measure for voltage. Voltage is electrical pressure that forces current to flow in a conductor, such as a wire.
Voltage Regulator
A circuit that has a constant output voltage when the input voltage fluctuates.
Volt-Amp (VA)
A unit of measure for apparent power. See Apparent Power.


A unit of measure for real power: Watts = VA / Power Factor
See Square wave, PWM Sine Wave and Sine Wave.


Zero Transfer Time
The transfer time of an on-line UPS system from AC line power to battery power. Since an on-line UPS system is already converting from DC to AC as part of the double conversion process, the transfer time between power failure detection and power delivery to equipment during an outage is instantaneous (zero). While the transfer time of a line-interactive UPS system designed for network applications is extremely fast (two to four milliseconds), this short delay may cause some sensitive equipment to malfunction or shut down. See Line-Interactive UPS, On-Line Double-Conversion UPS, Transfer Time.

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