Glossary - Fact Book

The following list provides definitions of selected terms that are commonly used in the electricity generating industry.

Alternating current. Flow of electricity that constantly changes direction between positive and negative sides. Almost all power produced by U.S. electric utilities moves in a current that shifts direction at a rate of 60 times per second.

Ampere (amp). The rate at which an electric current flows through a wire at a resistance of one ohm when a potential of one volt is applied across the resistance.

Base-load plant. A power plant built to operate around the clock. Such plants typically have low operating costs and high capital costs and are best utilized by running continuously. Coaland nuclear-fueled plants are typical base-load plants.

Capacity factor. The ratio of the electric energy produced by a generating unit to the electrical energy that could have been produced at continuous full-power operations.

Circuit. A conductor or system of conductors through which an electric current is intended to flow.

Combined cycle unit. An electricity generating unit consisting of combustion turbine(s) combined with a steam turbine. The steam turbine is fueled by the waste exhaust heat from the combustion turbine(s).

Combustion turbine. A type of generating unit normally fired by oil or natural gas. The combustion of the fuel produces expanding gases, which are forced through a turbine, which produces electricity by spinning a generator.

Conductor. A wire or cable capable of carrying electricity.

Demand. The amount of power that must be supplied to a customer (i.e., a load). (See Peak Demand)

Direct current. Current that flows continuously in one direction. Not common in home use.

Electric company. The company that delivers electricity to a customer’s home or business through its system of poles, power lines, and other equipment.

Electricity supplier. An entity that sells electricity to customers (and, in Maryland, is licensed to do so by PSC).

Federal Energy Regulatory Commission (FERC). An independent commission responsible for regulating wholesale electric power transactions and interstate transmission and sale of natural gas for resale. FERC is the federal counterpart to state utility regulatory commissions.

Generation. The process of producing electrical energy.

Gigawatt-hour (GWh). A quantity of electrical energy equal to 1 million kilowatt-hours.

Heat rate. A measure of the thermal efficiency of a generating station. It can be computed by taking the total heat content of fuel burned for electricity generation (expressed in British thermal units, Btus) and dividing by the resulting net kilowatt-hour generation.

Independent System Operator (ISO). A voluntary organization designed to encourage open and non-discriminatory grid access. The ISO controls and operates the transmission systems of regional utilities so that generation companies can move (or wheel) power over the transmission lines. Actual ownership of the transmission facilities remains with the individual utilities, which continue to collect revenues and pay expenses related to the transmission assets.

Interchanges. Intercompany power flows between members of a power pool or power system, generally of a short-term nature.

Kilovolt (kV). A unit of electrical pressure equal to 1,000 volts.

Kilovolt amperes (kVa). Volts times amperes divided by 1000. kVa is actual measured power (apparent power) and is used for circuit sizing.

Kilowatt (kW). A unit of electrical power equal to 1,000 watts.

Kilowatt-hour (kWh). A quantity of electrical energy equal to 1 kilowatt acting for 1 hour.

Load. Kilowatt or megawatt demand placed on the electric system by consumers of power.

Load factor. The ratio of average demand to maximum (i.e., peak) demand for a time period.

Loop flow. The movement of electric power from generator to load by dividing along multiple paths; it especially refers to power flow along an unintended path that loops away from the most direct geographic path or contract path.

Megawatt (MW). A unit of electrical power equal to 1 million watts.

Mill. A measure of cost or revenue equal to one-tenth of one cent.

Ohm. A unit of resistance allows one ampere to flow at a potential difference of one volt.

Peak demand. The maximum demand on an electric system in a designated period of time (e.g., over a year, a month, or a season).

Peaking plants. Power plants that operate for a relatively small number of hours, usually during peak demand periods. Such plants usually have high operating costs and low capital costs.

Power factor. The ratio of real power to apparent power to determine how much of the current contributes real power to the load. A power factor of one (i.e., 1.00) indicates that all current is contributing power to the load. The importance of power factors close to unity is to actually deliver the power to the load (i.e., maximize the amount of usable power from the least amount of input line current), instead of having the power reflected back to the source.

Regional Transmission Organization (RTO). An RTO controls, operates, and may independently own the transmission facilities historically held by a region’s vertically integrated public and private utilities. An RTO is an organization independent of the transmission facility owners. The RTO operates the high-voltage transmission grid to provide non-discriminatory access to the grid so that the lowest-priced wholesale power can be delivered to wholesale customers (e.g., load serving entities), while the owners still market and sell power.

Reliability councils. Regional organizations formed by the electric utilities to coordinate utilities’ generation and transmission systems and monitor the availability of electric services.

Retail competition. Permitting end-use customers to contract directly with suppliers for their electric or gas service, while transmission and distribution companies provide for delivery of the service.

Reserve margin. Total system generating capacity minus annual system peak demand, divided by the annual system peak demand, expressed as a percent.

Standard offer service (SOS). Electricity service that is provided to customers who do not choose an electricity supplier. Maryland’s SOS service is based on competitive wholesale market rates.

Time of use rates. A utility rate structure that charges higher rates during peak hours of the day in an effort to shift peak period demand to off-peak hours.

Transmission and distribution. The process of delivering electricity from generation plants to homes and businesses.

Volt. A unit of electrical pressure that can cause a current of one ampere to flow through resistance of one ohm.

Wheeling. A transaction between two utilities that are not electrically connected requiring the use of the transmission facilities of a third party. Often, the third party considers these transactions as buy/sell arrangements.

Watt. The electrical unit of power or rate of doing work. 746 watts is equivalent to one horsepower.

Watt-hour. An electric energy unit of measure that is equal to 1 watt of power supplied or taken steadily from an electric circuit for 1 hour.

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