Data Center Power Glossary – 100 Terms From BTM to WUE

Sit in on one data center planning meeting and you will hear a dozen acronyms before the coffee gets cold. BTM, PUE, N+1, STS, kVA, MMBtu. Nobody stops to define them. This data center power glossary collects the terms that show up most often in site studies, equipment specs and utility conversations. What do the most common data center power terms actually mean, in plain English?

The 100 terms below cover the full path power takes into a data center: the units used to measure it, the grid and market rules that deliver it, the on-site generators and fuel that can replace it, the electrical gear that distributes it, the redundancy that protects it, and the cooling and water systems that carry its heat back out. Terms are grouped by topic and listed alphabetically inside each group, so you can scan a section or jump to the word you just heard.

Units and Electrical Basics

Alternating current (AC)

Current that reverses direction many times per second. The grid delivers AC, at 60 hertz in North America.

Ampere (amp)

The unit of electrical current, the rate at which charge flows through a conductor. Wires and breakers are rated in amps.

Apparent power (kVA)

Voltage times current, in kilovolt-amperes. Transformers, generators and UPS systems are often rated in kVA rather than kW.

British thermal unit (Btu)

The heat needed to warm one pound of water by one degree Fahrenheit. The EIA's energy unit guide puts one kWh at 3,412 Btu.

Direct current (DC)

Current that flows in one direction only. Batteries, solar panels and the circuits inside servers all run on DC.

Frequency

How many times per second AC reverses, measured in hertz (Hz). Frequency sags when demand outruns generation, so operators watch it constantly.

Harmonics

Distorted current waveforms created by electronic loads such as server power supplies. They can overheat transformers and neutral conductors.

Kilowatt-hour (kWh)

Energy equal to one kilowatt delivered for one hour. Utility bills, battery capacity and annual facility use are counted in kWh or MWh.

Megawatt (MW)

One million watts, a rate of power. Think of MW as how fast energy flows and MWh as how much flowed.

Power factor

The ratio of real power (kW) to apparent power (kVA). Modern server power supplies run close to 1.0; motors often run lower.

Reactive power (kVAR)

Power that sustains the magnetic and electric fields in motors and lines. It does no useful work, yet still loads wires and transformers.

Real power (kW)

The power that actually does work: running servers, pumps and fans. Generators are usually sized and sold in kW or MW.

Therm

A unit of heat equal to 100,000 Btu. Many gas utilities bill homes and small businesses by the therm.

Three-phase power

Three AC waveforms offset by 120 degrees, delivering smoother, more efficient power. Nearly all data center equipment uses it.

Voltage

The electrical pressure that pushes current through a circuit. Higher voltage moves the same power with less current and smaller conductors.

Watt

The basic unit of power, one joule per second. A kilowatt is a thousand watts; a gigawatt is a billion.

Grid, Utility and Market Terms

Ancillary services

Grid support such as frequency regulation and spinning reserves, bought to keep supply and demand balanced second to second.

Capacity market

A market where grid operators pay generators and demand response providers to be available in future years, separate from payment for energy.

Demand charge

A utility charge based on the customer's highest power draw in a billing period, measured in kW, on top of energy charges.

Demand response

Paying customers to cut or shift load when the grid is stressed or prices spike.

Distribution

The lower-voltage network of feeders, poles and transformers that carries power from substations to homes and businesses.

ERCOT

The Electric Reliability Council of Texas, which operates the grid serving most of Texas and runs its wholesale power market.

FERC

The Federal Energy Regulatory Commission, which regulates interstate transmission and wholesale electricity markets in the United States.

Front of the meter

Generation and storage on the utility side of a customer's meter, selling into the grid rather than serving one site directly.

Interconnection queue

The line of projects waiting for a utility or grid operator to study and approve their connection to the system.

ISO/RTO

Independent System Operator or Regional Transmission Organization: a nonprofit that runs a regional grid and wholesale market, such as PJM or MISO.

Load study

A utility engineering study showing whether the system can serve a new load, which upgrades are needed and roughly what they cost.

NERC

The North American Electric Reliability Corporation, which writes and enforces reliability standards for the bulk power system.

Power purchase agreement (PPA)

A long-term contract to buy electricity from a specific generator at an agreed price, commonly running 10 to 20 years.

Substation

A fenced yard of transformers, breakers and switches that changes voltage and routes power between transmission and distribution.

Tariff

A utility's regulator-approved rate schedule and rules, setting prices, terms and obligations for each class of customer.

Transmission

High-voltage lines and towers that move bulk power over long distances from power plants to substations near load centers.

On-Site Generation and Fuel

For how these technologies compare in practice, see our guide to on-site power generation options.

Aeroderivative turbine

A gas turbine adapted from jet engine designs, lighter and quicker to start than a heavy industrial frame turbine.

Behind the meter (BTM)

Power produced on the customer's side of the utility meter, as in Enverus's definition: on-site generators, solar, fuel cells or batteries.

Black start

The ability to start a generator or power system from a total blackout without help from the outside grid.

Combined cycle

A plant that routes hot gas turbine exhaust through a heat recovery boiler to drive a steam turbine, raising overall efficiency.

Combined heat and power (CHP)

Making electricity and useful heat from the same fuel, such as using engine exhaust to drive chillers or heat buildings.

Firm transportation

Reserved pipeline capacity delivered without interruption except in extraordinary events. It costs more than interruptible service, which can be cut.

Flare gas

Natural gas burned at a well or plant because no pipeline, market or on-site use is available for it at the time.

Fuel cell

A device that turns fuel such as natural gas or hydrogen into electricity electrochemically, without combustion and with few moving parts.

Gas turbine

An engine that compresses air, burns fuel in it and expands the hot gas through turbine blades to spin a generator.

Heat rate

Fuel energy, in Btu, needed to make one kWh. Lower is better; a perfect machine would need 3,412 Btu/kWh.

Mcf

One thousand cubic feet of natural gas, the common volume unit in gas sales. MMcf means one million cubic feet.

Methane number

A gas fuel's resistance to engine knock. Pure methane scores 100; heavier hydrocarbons such as propane pull the number down.

Microgrid

Loads and local energy sources controlled as one system that can run connected to the utility or islanded on its own.

MMBtu

One million Btu, the standard energy unit for natural gas pricing. One Mcf of typical pipeline gas holds about 1.04 MMBtu.

Prime power

A generator rating for serving as the main power source, for unlimited hours at varying load, under ISO 8528 definitions.

Reciprocating engine

A piston engine, usually burning natural gas or diesel, that drives a generator. Efficient, modular and quick to start.

Simple cycle

A gas turbine plant with no heat recovery, so exhaust heat leaves through the stack. Quicker and cheaper to build, but less efficient.

Standby power

A generator rating for emergency use during utility outages, with limited yearly run hours and no overload allowance.

Power Distribution Inside the Facility

Automatic transfer switch (ATS)

A switch that senses a utility failure, moves the load to a generator, then moves it back once utility power returns.

Busway

Enclosed copper or aluminum bars that carry power overhead or along rows, with plug-in tap boxes so racks can be fed quickly.

Circuit breaker

A switch that opens automatically to stop current during an overload or short circuit, protecting wiring and equipment downstream.

Flywheel

A spinning mass that stores energy and releases it for seconds, bridging the gap until generators start.

Generator paralleling

Synchronizing several generators onto one electrical bus so they share load, adding capacity and redundancy.

Medium voltage (MV)

Roughly 1 kV to 35 kV, used across large sites because it moves big blocks of power with modest cable sizes.

One-line diagram

A simplified drawing of the electrical path from utility and generators through switchgear, transformers and UPS to the loads.

Power distribution unit (PDU)

Equipment that feeds power to racks. Floor PDUs hold transformers and breakers; rack PDUs are the outlet strips inside cabinets.

Remote power panel (RPP)

A breaker panel placed near rows of racks, fed from a PDU, with branch circuits running out to individual cabinets.

Ride-through

Equipment's ability to keep running through a brief voltage sag or interruption, usually supplied by UPS batteries or internal capacitors.

Selective coordination

Setting breakers so that only the device closest to a fault trips, keeping the rest of the facility energized.

Static transfer switch (STS)

A solid-state switch that moves a load between two power sources within a few milliseconds, much faster than a mechanical switch.

Switchgear

Assemblies of breakers, switches, relays and meters that control, protect and isolate the major electrical equipment at a site.

Transformer

Equipment that raises or lowers voltage through magnetic coupling between windings, with no moving parts.

Uninterruptible power supply (UPS)

A system that powers critical loads instantly during an outage, from batteries or flywheels, until generators take over.

VRLA battery

Valve-regulated lead-acid battery, a sealed type long used in UPS systems. Cheaper upfront than lithium-ion, but shorter lived.

Reliability and Redundancy

2N

Two complete, independent power or cooling paths, each able to carry the full load alone. Expensive, but it survives losing either one.

Availability

The share of time a system can deliver power or service, usually written as a percentage or as a count of nines.

Commissioning

Structured testing that proves equipment and systems work as designed, ending with integrated tests under load before the site goes live.

Concurrently maintainable

A design in which any component can be taken out for planned maintenance without shutting down the IT load.

Fault tolerant

A design that keeps the IT load running through any single unplanned equipment failure, without anyone having to intervene.

MTBF

Mean time between failures: the average operating time between breakdowns of repairable equipment, used in reliability models.

N+1

Enough units to carry the load (N) plus one spare. Six 2 MW engines serving a 10 MW load is N+1.

Nines

Shorthand for availability. Four nines, or 99.99 percent, allows about 53 minutes of downtime per year.

Single point of failure

Any one component whose failure takes down the whole system, such as a lone transformer.

Tier

The Uptime Institute's four-level rating of data center infrastructure, from basic capacity (Tier I) up to fault tolerant (Tier IV).

Cooling and Water

Adiabatic cooling

Pre-cooling air by wetting pads only on hot days, so a dry cooler uses far less water than a cooling tower.

Chiller

A machine that pulls heat out of a water loop with a refrigeration cycle, supplying chilled water to air handlers and coils.

Containment

Barriers that separate hot and cold aisles so server exhaust does not mix with intake air, which makes cooling more efficient.

Coolant distribution unit (CDU)

A pump and heat exchanger package that circulates a clean, controlled secondary coolant loop to liquid-cooled racks.

Cooling tower

Equipment that rejects heat by evaporating part of the circulating water into passing air. Efficient, but thirsty.

CRAC

Computer room air conditioner: a room cooling unit with its own refrigerant compressor, rejecting heat to an outdoor condenser.

CRAH

Computer room air handler: a fan and coil unit cooled by chilled water from a central plant, with no compressor inside.

Delta T

The temperature difference across equipment, such as server inlet versus outlet air, or chilled water supply versus return.

Direct-to-chip cooling

Liquid cooling that pumps coolant through cold plates mounted on processors, carrying heat away before it reaches room air.

Dry cooler

A closed-loop radiator with fans that rejects heat to the outside air without evaporating any water.

Economizer

A mode that uses cool outside air or water to cool the facility, letting chillers rest whenever the weather allows.

Immersion cooling

Submerging servers in a nonconductive fluid. Single-phase fluids stay liquid; two-phase fluids boil at the chip and condense above.

PUE

Power usage effectiveness: total facility energy divided by IT energy. A PUE of 1.5 means 0.5 kWh of overhead per IT kWh.

Rack density

Power drawn per server rack, in kW. It sets how much cooling and electrical capacity each row of cabinets needs.

Ton of refrigeration

Cooling capacity equal to 12,000 Btu per hour, which works out to about 3.52 kW of heat removal.

WUE

Water usage effectiveness: annual site water use in liters divided by IT energy in kWh. Lower is better.

Storage, Renewables and Emissions

Battery energy storage system (BESS)

Batteries, inverters, controls and cooling packaged together to store electricity and deliver it back on demand.

Curtailment

Deliberately cutting output from wind or solar, or cutting a load, because the grid cannot absorb it or prices make it uneconomic.

Depth of discharge

The share of a battery's capacity used in a cycle. Deeper cycling delivers more energy but can shorten battery life.

NOx

Nitrogen oxides, pollutants formed when fuel burns hot. Limiting them is a main focus of generator air permits.

Renewable energy certificate (REC)

A tradable certificate representing the environmental attributes of one MWh of renewable generation.

Round-trip efficiency

The share of energy put into storage that comes back out. Lithium-ion systems commonly return around 85 percent.

Scope 2 emissions

Indirect greenhouse gas emissions from the electricity, steam or heat a company buys, as defined by the GHG Protocol.

Selective catalytic reduction (SCR)

Exhaust treatment that injects ammonia or urea over a catalyst, converting NOx into nitrogen and water vapor.

Keep This Data Center Power Glossary Next to the One-Line

Most data center power terms fall into a handful of families: how power is measured, sold, made on site, distributed, backed up and cooled. Once you know which family a word belongs to, the conversation gets much easier to follow.

Bookmark this page for your next utility call or equipment review. When an unfamiliar acronym shows up in a spec sheet, check it here first, then ask the engineer what it means for your site. That second question is usually where the useful answer lives.

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