AC Efficiency

The ratio of the output power from the UPS to the input power from the AC utility in Line mode.

AC Utility

The electric power furnished by an electric power plant


The unit of battery capacity (Ampere-hour).

Ampere (Amp or A)

The unit of for electric current.

Automatic Voltage Regulator (AVR)

An AVR regulates voltage to the appropriate level.

Backup Time

The time provided by battery in a UPS system when AC utility loss.

Battery Charging Current

The current to charging the battery that is regulated from the AC utility by the circuit inside the UPS.


Complete loss of power. Some literature considers a voltage drop below about 80V to be a blackout as well since most equipment will not operate below these levels.

Sag or Brownout

Decrease in voltage levels which can last for periods ranging from fractions of a second to hours. Can be caused by heavy equipment coming on line such as shop tools, elevators, compressors etc. Also occurs when utility companies deliberately do this to cope with peak load times.

Backup Time/Runtime

The duration that UPS supplies backup power to the electrical appliances while power failure. The unit of backup time is minute and calculates backup time from black out happening to battery used up.

Buck and Boost

A function to adjust the output of UPS. When the voltage is under normal condition then the UPS will boost the voltage higher. When the voltage is over normal condition then the UPS will buck the voltage lower.

Cold start function

The UPS can be started from battery directly without AC utility. Some people also name it as DC start.


UPSs can often communicate with computer equipment in order to allow the UPS to trigger a controlled automatic shutdown of the computer. This is usually done when the mains has failed and the UPS battery power is close to being used up.

DC Efficiency

The ratio of the output power from the UPS to the input power from the batteries in Battery Mode.

DC Voltage

Normally, it is used for the battery voltage.

Electromagnetic Interference (EMI)

Unwanted noise during the operation of a power supply or other electrical or electronic equipment.

Fast Charging

It is the fast charging battery system to speed up charging time. For this case, the designer must be expert and use high technology in controlling charging system so battery does not damage.


The number of completed cycles in one second. It is defined as Hertz (Hz). There are 50 or 60 Hz power systems in the world.

Harmonic Distortion

Distortion of an electrical sine wave caused by the power supplies of certain electronic devices. A mathematical representation of a distortion of a pure sine waveform.

Hot-swappable Batteries

Allow the user to easily exchange UPS batteries without turning off the UPS.

Input Power Cord

The cord for the AC input of UPS. Normally, there are three wires of Line-Neutral-Ground in it

Intelligent Battery Management (IBM)

The intelligent battery management will manage charging and discharging to get longer backup times and prolong battery life.


A power supply which produces an AC output, usually from a DC input.


Two circuits that are completely electrically separated with respect to DC potentials, and almost always also AC potentials. In power supplies, it is defined as the electrical separation of the input and output via the transformer.

Isolation Voltage

The maximum AC or DC voltage which maybe continuously applied from input to output and/or chassis of a power supply.

Leakage Current

A term relating to current flowing between the AC supply wires and earth ground. The term does not necessarily denote a fault condition. In power supplies, leakage current usually refers to the 60 Hertz current which flows through the EMI filter capacitors which are connected between the AC lines and ground (Y caps).

Line Interactive UPS

A type of uninterruptible power supply that is in standby mode most of the time but contains a transformer that helps condition and adjusts the voltage until the voltage exceeds a set limit. If the voltage limit is exceeded, the UPS can support the load until the voltage returns to within the limits or the battery is drained.

Manual Bypass Switch

A manually operated transfer switch used to bypass the major electronics in the UPS. When it is enabled then the service engineer can have the repair service without power interruption.


Metal Oxide Varistors are added to circuits in order to control spikes. These are common in Power Strips. If you see more than two, you likely have a fairly decent Power Strip. They look like largish disk capacitors.

No Load Shut Down

UPS will shutdown itself when the utility fails and there is no load connected to UPS. This feature is to prolong battery life.

Nominal Voltage

A standard value assigned to a circuit for the purpose of conveniently designating its voltage class.

Online (double conversion) UPS

A type of uninterruptible power supply that is constantly converting the incoming power from AC to DC and then inverting DC back to AC. This double conversion process prevents power disturbances from reaching protected loads. The UPS batteries will support the load if conditions exceed a set limit.

Over-voltage Protection

A circuit which either shuts down the power supply or crowbars the output in the event of an over-voltage condition.

Power failure

Total loss of utility power: Causes electrical equipment to stop working.

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 line and load changes with minimal dissipation.

Power Factor (PF)

The ratio of true power to apparent power in an AC circuit. In power conversion technology, power factor is used in conjunction with describing the AC input current to the power supply.

Rack-Mount UPS

UPS that can be mounted in a rack case.

Radio Frequency Interference (RFI)

It is undesirable noise produced by a power supply or other electrical or electronic device during its operation. In power supply technology, RFI is usually taken to mean the same thing as EMI.


A socket or outlet into which a plug can be inserted to make an electrical connection.

Serial Port

A special cable plugs into the UPS and the computer's serial (RS232) port.


A tremendous increase in voltage over a very short period of time often caused by a direct lightning strike on a power line or when power returns after a blackout.

Standby UPS

A type of uninterruptible power supply that remains in standby mode offering little or no power conditioning unless the voltage exceeds a set limit. If the voltage limit is exceeded, the UPS can support the load until the voltage returns to within the limits or the battery is drained.


A substantial increase in voltage lasting a small fraction of a second, often caused when high powered appliances such as air conditioners are switched off.

Surge Protector

Circuitry consisting of MOVs, capacitors, rod-core inductors etc. for suppressing surges and spikes usually embedded in a power strip.

Transfer Time

The amount of time it takes a stand-by or off-line type UPS to sense a power interruption and switch from utility output to inverter output. Normally expressed in milliseconds.

Uninterruptible Power Supply (UPS)

A device which maintains a continuous supply of electric power to connected equipment by supplying power from a separate source when utility power is not available.

USB Port

A USB cable connects to the USB port on a computer and to the UPS.

User-Replaceable Batteries

User replaceable batteries allow the user to easily exchange UPS batteries. Normally, the battery lifetime is close to 2~3 years.

Volt (V)

The unit of measure for voltage.

Volt-Ampere (VA)

Voltage (V) multiplied by the current (ampere); apparent power.

Voltage sag

Transient (short term) under-voltage: Causes flickering of lights.

Voltage spike

Transient (short term) over-voltage i.e. spike or peak: Causes wear or acute damage to electronic equipment.

Watt (W)

The unit of measure for true power. Watts = VA x Power Factor.

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