An uninterruptible power supply (UPS) is an electrical device that provides emergency power to connected equipment when the main power source (typically utility power) fails. It conditions incoming power to ensure clean and uninterrupted power, protects devices from power problems and enables seamless system shutdown during complete outages.

With aging electrical grids, rising power demand, severe weather, faulty wiring and disruptive devices connected to AC lines, equipment is under constant threat from power problems, making a UPS system an important investment. A UPS system is especially useful for networking equipment and other devices that can lose data when power is unexpectedly lost. With a wide range of cost-effective models available, a UPS system is an essential investment to prevent damage, data loss and downtime caused by power problems. 

1. Determine your power requirements

• Calculate the load: Identify the total wattage of all devices you plan to connect to the UPS. This includes computers, monitors, networking equipment, and other peripherals.
• VA rating: Understand the Volt-Ampere (VA) rating, which is the product of voltage and current. Ensure the UPS can handle the combined VA rating of your devices.

2. Select the UPS topology

• Standby UPS: Suitable for basic protection against power outages and surges. Ideal for home use and small office setups. 
• Line-Interactive UPS: Provides better voltage regulation and is suitable for environments with frequent power fluctuations. 
• Online UPS: Offers the highest level of protection by continuously converting incoming AC power to DC and then back to AC. Ideal for critical applications like data centers and medical equipment. 

3. Consider runtime

• Battery backup duration: Determine how long you need the UPS to provide power during an outage. This depends on the battery capacity and the load connected.
• Extended runtime options: Some UPS models allow for additional battery packs to extend runtime. 

4. Evaluate form factor

• Desktop vs. rackmount: Choose between desktop models for individual workstations or rackmount models for server rooms and data centers. 

5. Brand and warranty

• Reputable brands: Consider UPS systems from well-known manufacturers with good reviews and reliable customer support. 
• Warranty and support: Check the warranty period and the availability of technical support and service. 

6. Check for additional features

• Sine wave output: Ensure the UPS provides a pure sine wave output, especially if you are protecting sensitive equipment.
• Hot-swappable batteries: Look for UPS systems with hot-swappable batteries for easy maintenance without downtime.
• Network management: Consider UPS models with network management capabilities for remote monitoring and control. 

See more considerations and additional features below.

Additional features to consider:

UPSs are given a power rating in volt-amperes (VA); this rating represents the maximum load that a UPS can support, but it shouldn’t match exactly the power load you have. To allow room for growth, the best practice is to choose a UPS with a VA rating that is 1.2x the total load you need it to support. If your UPS will be supporting motors, variable-speed drives, medical imaging devices or laser printers, add more VA capacity to your requirements to account for the high power inrush that occurs when those devices startup.

Companies that are anticipating rapid growth should use a higher multiplier than 1.2x. Newer server hardware tends to have higher power requirements than older models, so factoring in additional VA will account for adding more and newer equipment.

Alternating current (AC) An electric current that reverses its direction at regularly recurring intervals, as opposed to direct current, which is constant. Usually in a sine wave pattern, for optimal transmission of energy.

Ampere (A or Amp) The unit of measure for the rate of flow of electricity, analogous to gallons per minute. VA x 0.7 (power factor) = watts

Apparent power Applied voltage multiplied by current in an AC circuit which doesn’t take the power factor into account. Unit is volt amperes (VA).

Arc Sparking that results when undesirable current flows between two points of differing potential due to leakage through the intermediate insulation or a leakage path due to contamination.

Audible noise A measure of the noise emanating from a device at audible frequencies.

Backup time The amount of time the battery in a UPS is designed to support the load.

Balanced load AC power system using more than two wires, where the current and voltage are of equal value in each energized conductor.

Battery string A group of batteries connected together in a series.

Blackout A zero-voltage condition lasting for more than two cycles. Also known as a power outage or failure.

British Thermal Unit (BTU) Used to measure heat dissipation and is the amount of energy required to raise one pound of water one degree Fahrenheit. One pound of water at 32°F requires the transfer of 144 BTUs to freeze into solid ice.

Brown field An existing data center—often with limited possibilities for sustainable and energy-efficient designs.

Brownout A steady state of low voltage, but not zero voltage. Brownouts often occur during summer months when energy use is high.

Canadian Standards Association (CSA) An independent Canadian organization that tests for public safety, similar to the function of Underwriters Laboratories (UL) in the U.S. 

Capacitor An electronic component that can store an electrical charge on conductive plates.

Charger An electronic component in a UPS that provides regulated DC voltage to recharge batteries. 

Cloud computing Internet- (cloud-) based development and use of computer technology. This new supplement, consumption and delivery model for IT services typically involves the provision of dynamically scalable, and often virtualized, resources as a service over the Internet. 

Common mode noise An undesirable voltage that appears between the power conductors and ground.

Commercial power The power supplied by local utility companies which can vary drastically in quality throughout the U.S. depending on location, weather and other factors.

Converged infrastructure The combination of server, storage, networking, virtualization and sometimes other resources into an integrated solution that is managed as a whole rather than through separate management systems.

Converter A device that delivers DC power when energized by a DC source. It’s also a section of a switching power supply that performs the actual power conversion and final rectification.

Crest factor Usually refers to current. It’s the mathematical relationship between RMS and peak current. A normal resistive load will have a crest factor of 1.4142, which is the normal relationship between peak and RMS current. A typical PC will have a crest factor of 3. Unrelated to toothpaste.

Critical equipment Equipment such as computers, communications systems or electronic process controls, which must be continuously available.

Delta connection A circuit formed by connecting three electrical devices in series to form a closed loop; most often used in threephase connections. 

Derating A reduction of some operating parameters to compensate for a change in one or more other parameters. In power systems, the output power rating is generally reduced at elevated temperatures.

Direct current (DC) An electric current in which the flow of electrons is in one direction, such as supplied by a battery.

DC distribution (DCD) A module in a DC power system that distributes DC power to the loads. It also provides protection for the load cables.

DC power system An AC to DC power supply with integrated control and monitoring, and standby batteries designed to supply no-break DC power (usually 24V or 48V) to telecommunications and IT network equipment.

Double conversion A UPS design in which the primary power path consists of a rectifier and inverter. It isolates the output power from all input anomalies such as low voltage surges and frequency variations.

Downtime The time during which a functional unit can’t be used because of a fault within it or the environment.

Dry contacts Dry contact refers to a contact of a relay which does not make or break a current.

Efficiency The ratio of output to input power. Generally measured at full-load and nominal line conditions. If the power efficiency of a device is 90 percent, you get back 90 watts for every 100 you put in, and the rest is mainly dissipated as heat from the filtration process.

Electrical line noise Radio frequency interference (RFI), electromagnetic interference (EMI) and other voltage or frequency disturbances.

Electromagnetic interference (EMI) Electrical interference that can cause equipment to work improperly, EMI can be separated into conducted EMI (interference conducted through cables out of the UPS) and radiated EMI (interference conducted through the air).

Energy Saver System (ESS) Innovative technology from Eaton that enables select UPS models to operate at 99 percent efficiency without compromising reliability—not to be confused with inferior “eco” modes.

ePDU A power distribution unit that mounts to rack enclosures and distributes power to connected devices via a wide variety of output receptacles.

Federal Communications Commission (FCC) A U.S. federal regulating body whose new EMI limitations are affecting the design and production of digital electronics systems and their related subassemblies.

Flooded batteries A form of battery where the plates are completely immersed in a liquid electrolyte.

Frequency The number of complete cycles of AC voltage that occur during one second (Hz). In North America, electrical current is supplied mainly at 60 Hz, or 60 cycles per second.

Green field A new data center with many possibilities for sustainable and energy-efficient designs.

Ground A conducting connection, whether intentional or accidental, by which an electric circuit or equipment is connected to the earth, or to some conducting body of relatively large extent that serves in its place.

Harmonics A sinusoidal component of an AC voltage that’s multiple of the fundamental waveform frequency. Certain harmonic patterns may cause equipment problems.

Harmonic distortion Regularly appearing distortion of the sine wave which is converted into a complex waveform at a multiple of the fundamental frequency.

Hertz (Hz) A unit of frequency equal to one cycle per second.

High efficiency mode A mode of UPS operation that cuts energy usage and operating costs.

High voltage (HV) In the context of UPS products, high voltage is anything ≥200V: 200V, 208V, 220V, 230V, 240V, 250V, 480V and 600V.

High voltage spike Rapid voltage peaks up to 6,000 volts. Hot swappable The ability to change a module without taking the critical load off the UPS. Also see “user replaceable.”

Insulated gate bipolar transistor (IGBT) A three-terminal power semiconductor device, noted for high efficiency and fast switching. It switches electric power in many modern appliances such as electric cars, trains and UPSs.

Impedance The total opposition to alternating current flow in an electrical circuit.

Input voltage range The voltage range within which a UPS operates in “normal” mode and doesn’t require battery power.

Inrush current The maximum, instantaneous input current drawn by an electrical device when first turned on. Some electrical devices draw several times their normal full-load current when initially energized.

Inverter UPS assembly that converts internal DC power to output AC power to run the user’s equipment. When the inverter is supporting 100 percent of the load at all times, as with an online UPS, there is no break from utility to battery power.

Kilovolt ampere (kVA) A common measurement of equipment capacity equaling 1000 volt-amperes. An approximation of available power in an AC system that does not take the power factor into account.

Kinetic energy The energy an object possesses because of its motion.

Line conditioner A device intended to improve the quality of the power that’s delivered to electrical load equipment. A line conditioner is generally designed to improve power quality (e.g., proper voltage level, noise suppression, transient impulse protection, etc.).

Line interactive An offline UPS topology in which the system interacts with the utility line to regulate the power to the load. Provides better protection than a standby system but isn’t as fully prepared against irregularities as a full double-conversion system, making it the "Goldilocks" of UPS topologies.

Linear load AC electrical loads where the voltage and current waveforms are sinusoidal. The current at any time is proportional to voltage.

Lithium-ion battery A lightweight battery with high energy density. Its electrodes are made of lightweight lithium and carbon. Eaton utilizes a combination of lithium phosphate that creates a stable and safe battery for UPS applications. This type of chemistry does not create oxygen as an off-put should there be a thermal event, eliminating the harsh igniting potential of some lithium-ion battery chemistries. 

Load The equipment connected to and protected by a UPS. 

Low voltage (LV) In the context of UPS products, low voltage is anything <200V (100V and 120V).

Maintenance bypass An external wiring path to which the load can be transferred to upgrade or perform service on the UPS without powering down the load. 

Make-before-break Operational sequence of a switch or relay where the new connection is made prior to disconnecting the existing connection, that’s also know as soft-load-transfer switching.

Modbus A serial communications protocol that’s the most commonly available means of connecting industrial electronic devices. It allows for communication between many devices connected to the same network.

Network transient protector UPS feature that isolates networks, modems and cables from power threats, including surges and spikes.

Noise (1) A disturbance that affects a signal; it can distort the information carried by it. (2) Random variations of one or more characteristics of any entity, such as voltage, current or data. (3) Loosely, any disturbance tending to interfere with normal operation of a device.

Nominal output voltage The intended, ideal voltage of any given output.

Non-linear load AC electrical loads where the current is not proportional to the voltage. Non-linear loads often generate harmonics in the current waveform that lead to distortion of the voltage waveform.

Offline Any UPS that doesn’t fit the definition of online. Line-interactive and standby topologies are offline.

Ohm The unit of measurement for electrical resistance or opposition to current flow.

Ohm’s Law The voltage (E) is equal to the current (I) times the resistance (R). The formula is written: E=IR.

Online A UPS that provides power to the load from its inverter 100 percent of the time, regulating BOTH voltage and frequency, usually using double-conversion topology.

Orderly shutdown The sequenced shutdown of units comprising a computer system to prevent damage to it and subsequent corruption or loss of data.

Output waveform (UPS) The shape of the graph of alternating current on the output side of a UPS. The highest quality of an output waveform from a UPS is the sine wave, but, some UPSs provide step waves or modified sine waves.

Parallel operation The ability of UPSs to be connected so the current from corresponding outputs can be combined into a single load.

Partition A logical division of a hard disk created to have different operating systems on the same hard disk or to create the appearance of having separate hard drives for file management, multiple users, or other purposes.

Peak demand The highest 15- or 30-minute demand recorded during a 12-month period.

Phase The time relationship between current and voltage in AC circuits.

Plenum cable Cable that’s laid in the plenum spaces of buildings to facilitate air circulation for heating and air conditioning systems. The plenum space is typically used to house computer and telephone network communication cables. Cable that runs between floors in non-plenum areas is rated as riser cable.

Plug and play An electrical device that doesn’t require extensive setup to operate.

Power factor (PF) The ratio of real power to apparent power. Watts divided by VA. Most power supplies used in communication and computer equipment have a power factor of 0.9. (PF = 0.9) VA x PF = W W/PF = VA

Power sag Low voltage (below nominal 120 volts).

Power surge High voltage (above nominal 120 volts).

Pulse width modulation (PWM) A circuit used in switching regulated power supplies where the switching frequency is held constant and the width of the power pulse is varied, controlling both lines and load changes with minimal dissipation.

Rackmount Ability to mount an electrical assembly into a standardized rack. Generally stacked up to 42U and 19 inches wide.

Rack unit (U) A unit of height measurement in a rack enclosure. A U is equivalent to 1.75 inches

Rail kit A set of metal brackets that allows the installation of a UPS or extended battery module in a 2- or 4-post rack.

Rectifier A UPS component that converts incoming AC power to DC power for feeding the inverter and charging the battery.

Rectifier magazine (RM) A module in the DC power system used to connect the rectifiers in the power system.

Redundancy The ability to connect units in parallel so if one fails the other(s) will provide continual power to the load. This mode is used in systems when power failure can’t be tolerated.

Relay communication Communication between a UPS and a computer through the opening and closing of solid-state relays that are pre-defined to indicate UPS status.

Root mean square (RMS) A modified average. Averaging a sine wave would give a zero, so to obtain meaningful values, the wave is first squared (S), then averaged over one period (M) and finally the square root taken (R). In a sine wave, the factor between RMS and peak is the square root of two.

RS-232 The standard for serial interfaces (serial refers to the eight bits of each character successively sent down one wire) used by most computers, modems and printers.

Server room Dedicated computer room with some power and cooling, typically within an office environment. Minimal redundancy for power and cooling distribution. Singular source of power and cooling. (451 Research)

Server closet Small room or closet with little to no redundancy power and cooling distribution. Singular source of power and cooling. (451 Research)

Simple Network Management Protocal (SNMP) A User Datagram Protocol (UDP)-based network protocol. It’s used mostly in network management systems to monitor network-attached devices for conditions that warrant administrative attention.

Sine wave A mathematical function that plots three qualities of an electrical signal over time: amplitude, frequency and phase. Clean, uninterrupted power is represented by a sine waves, which can also resemble ocean waves, though they're rarely perfect.

Single phase Power system with one primary waveform. Lower-capacity distribution of power using only one portion of a power source that’s three-phase, like what's supplied by most electric utilities. Used for heating and lighting, no large motors or other heavy-drain devices.

Sliding demand Calculating average demand by averaging demand over several successive time intervals, advancing one interval at a time.

Standby UPS type that "stands by," waiting for a power problem from the utility company and rapidly switching to UPS battery power to protect equipment against power failures, sags and surges.  

Static switch An electrical component in a UPS that turns power flow on and off on command without moving or mechanical components.

Step load An instantaneous change in the loading conditions presented to the output of a UPS.

Switching Frequency The rate at which the source voltage is switched in a switching regulator or chopped in a DC-to-DC converter.

Technischer Uberwachungs-Verein (TUV) An independent non-profit organization that tests and certifies electrical equipment for public safety in the U.S. and worldwide.

Thermal regulation Monitoring the temperature of the batteries to ensure proper charging.

Three phase Power supplied through at least three wires, each carrying power from a common generator but offset in its cycle from the other two. Used for heavy-duty applications.

Topology (UPS) The core technology of a UPS. Typically, a UPS is either standby, line interactive or online though other hybrid technologies have been introduced.

Total harmonic distortion (THD) How much the circuit voltage deviates from a perfect sine wave. When viewed on a meter, a poor voltage THD is most often manifested in a flat-topped waveform that comes from the inability of a power source to respond to the demands of highly nonlinear loads.

Transfer time The length of time it takes a UPS to transfer to battery power. Typically measured in milliseconds (ms).

Transformer A magnetic device that converts AC voltages to AC voltages at any level. An ideal transformer is a lossless device in which no energy is stored that requires no magnetic current.

Transient A temporary and brief change in a given parameter, typically associated with input voltage or output loading parameters.  

Unbalanced load An AC power system using more than two wires, where the current is not equal due to an uneven loading of the phases.

Underwriters Laboratories (UL) An independent non-profit organization that tests for public safety in the U.S. UL recognition is required for equipment used in some applications.

Uninterruptible power system (UPS) An electrical system designed to provide instant, transient-free backup power during power failure or fault. Some UPSs also filter and/or regulate utility power (line conditioning). 

User replaceable Capable of being replaced by an end user. Connected equipment may need to be shut down first. Also see “hot swappable.”

Variable Module Management System (VMMS) Innovative technology from Eaton that maximizes UPS efficiencies at low load levels while supplying the load with continuous double-conversion power.

Virtualization The creation of a virtual (rather than actual) version of something, such as an operating system, server, storage device or network resource. Operating system virtualization is the use of software to allow a piece of hardware to run multiple operating system images at the same time.

Volt/voltage (V) Electrical pressure that pushes current through a circuit. High voltage in a computer circuit is represented by 1, while low (or zero) voltage is represented by 0.

Volt amps (VA) The voltage applied to a given piece of equipment, multiplied by the current it draws. Not to be confused with watts, which are similar but represent the actual power drawn by the equipment, and can be somewhat lower than the VA rating.

Watts (W) The measure of real power. It’s the rate of doing electrical work. W x 1.3 = VA.

Wye connection A connection of three components made in such a manner that one end of each component is connected. It’s generally used to connect devices to a three-phase power system.