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PADD: See Petroleum Administration for Defense District (below).

Parallel Path Flow: As defined by North American Electric Reliability Council, this refers to the flow of electric power on an electric system's transmission facilities resulting from scheduled electric power transfers between two other electric systems. (Electric power flows on all interconnected parallel paths in amounts inversely proportional to each path's resistance.)

Particulate Matter (PM): Unburned fuel particles that form smoke or soot and stick to lung tissue when inhaled. A chief component of exhaust emissions from heavy–duty diesel engines.

Passive Solar Energy: Use of the sun to help meet a building's energy needs by means of architectural design such as arrangement of windows and materials such as floors that store heat, or other thermal mass.

Passive Solar System: A solar heating or cooling system that uses no external mechanical power to move the collected solar heat.

Peak Demand: Peak demand is the maximum electric load or demand during a specified period of time. See Peak Load (below).

Peak Load: The electric load that corresponds to a maximum level of electric demand in a specified time period. Peak periods during the day usually occur in the morning hours from 6 to 9 a.m. and during the afternoons from 4 to about 8 or 9 p.m. The afternoon peak demand periods are usually higher, and they are highest during summer months when air–conditioning use is the highest. Annual peaks generally occur on hot summer days.

Peak Load Power Plant: A power generating station that is normally used to produce extra electricity during peak load times.

Peaking Unit: A power generator used by a utility to produce extra electricity during peak load times.

Peat: A heterogeneous mixture of partly decomposed organic matter that has accumulated in a water saturated environment over a very long period of time. Peat geologically is considered a very young form of coal and has a heating value of 6,600 Btu/pound.

Pentanes Plus: A mixture of hydrocarbons, mostly pentanes and heavier, extracted from natural gas. Included are isopentane, natural gasoline, and plant condensate.

Per Capita: For each person.

Perm:The measurement of water vapor through different materials measured in perm–inch (mass of water vapor moving through a unit area in unit time).

Personal Income: Personal income is defined as the income that is received by a person from all sources. It is calculated as the sum of wage and salary disbursements, supplements to wages and salaries, proprietors' income with inventory valuation and capital consumption adjustments, rental income of persons with capital consumption adjustment, personal dividend income, personal interest income, and personal current transfer receipts, less contributions for government social insurance. Personal income is reported in current dollars (no adjustment is made for price changes).

Petrochemicals: Chemicals made from oil.

Petrochemical Feedstocks: Chemical feedstocks derived from petroleum principally for the manufacture of chemicals, synthetic rubber, and a variety of plastics. The categories reported are "Naphthas Less Than 401 Degrees Fahrenheit Endpoint" and "Other Oils Equal to or Greater Than 401 Degrees Fahrenheit Endpoint".

Petrodollars: Money paid to other countries for oil imported to the United States.

Petroleum: Oil as found in its natural state under the ground. A generic term that refers to oil and oil products in all forms such as crude oil, unfinished oil and similar terms.

Petroleum Administration for Defense District (PADD): The U.S. Department of Energy divides the United States into regions for planning purposes. The result is a geographic aggregation of the 50 States and the District of Columbia into five Districts, with PADD I further split into three subdistricts. A map is available. The PADDs include the States listed below:

Petroleum Coke: A residue high in carbon content and low in hydrogen that is the final product of thermal decomposition in the condensation process in cracking.

Photocell: A device that produces an electric reaction to visible radiant energy (light).

Photosynthesis: A process by which green plants change carbon dioxide into oxygen and organic materials. The energy for this process comes from sunlight.

Photovoltaic and Solar Thermal Energy: Energy radiated by the sun as electromagnetic waves that is converted into electricity by means of solar (photovoltaic) cells or concentrating (focusing) collectors.

Photovoltaic Cell: A semiconductor that converts light directly into electricity.

Pipeline: A line of pipe with pumping machinery and apparatus (including valves, compressor units, metering stations, regulator stations, etc.) for conveying a liquid or gas.

Plant Condensate: One of the natural gas liquids, mostly pentanes and heavier hydrocarbons, recovered and separated as liquids at gas inlet separators or scrubbers in processing plants.

Polychloronated Biphenyls (PCBs): A group of organic compounds used in the manufacture of plastics and formerly used as a coolant in electric transformers. In the environment, PCBs are highly toxic to aquatic life. They persist in the environment for long periods of time and are biologically accumulative.

Population–Weighted Degree Days: Heating or cooling degree days weighted by the population in the area in which the degree days are recorded. For example, to calculate the state population–weighted degree days, the state is divided into nine climatically similar divisions that are assigned weights based on the ratio of the population of the division to the total population of the state. Degree day readings for each division are multiplied by the corresponding population weight for each division, and these products are then summed to arrive at the state population–weighted degree day figure.

Pounds Per Square Inch: A unit of pressure, which is a unit measuring force per unit area. Also see psi.

Pour Point: Pour point is the point at which flow of the fuel ceases.

Power: The rate at which energy is produced, used, or converted from one form to another.

Power Authorities and Power Districts: Quasi–governmental agencies that perform all or some of the functions of a public utility.

Power Plant: A central station generating facility that produces energy.

Power Pool: 1) An entity established to coordinate short–term operations to maintain system stability and achieve least–cost dispatch. The dispatch provides backup supplies, short–term excess sales, reactive power support, and spinning reserve. Historically, some of these services were provided on an unpriced basis as part of the members' utility franchise obligations. Coordinating short–term operations includes the aggregation and firming of power from various generators, arranging exchanges between generators, and establishing (or enforcing) the rules of conduct for wholesale transactions. The pool may own, manage and/or operate the transmission lines ("wires") or be an independent entity that manages the transactions between entities. Often,the power pool is not meant to provide transmission access and pricing, or settlement mechanisms if differences between contracted volumes among buyers and sellers exist. 2) Two or more interconnected utilities that plan and operate to supply electricity in the most reliable, economical way to meet their combined load.

PPM (Parts Per Million): The unit commonly used to represent the degree of pollutant concentration where the concentrations are small.

Premium Gasoline: Gasoline having an antiknock index, i.e., octane rating, greater than 90. Includes both leaded premium gasoline as well as unleaded premium gasoline.

Pressurized Water Reactor (PWR): A nuclear power unit cooled by water that is pressurized to keep it from boiling when it reaches high temperatures.

Primary Energy: All energy consumed by end users, excluding electricity but including the energy consumed at electric utilities to generate electricity.

Primary Energy Resources: Resources that produce energy. Primary energy resources are petroleum, natural gas, coal, hydroelectric power, nuclear electric power, wood and waste, geothermal energy, solar thermal and photovoltaic energy, and wind energy. Electricity is a secondary energy resource because generating plants use primary energy resources to generate electricity.

Primary Fuel: Fuel consumed in the original production of energy, before conversion takes place.

Prime Mover: The engine, turbine, water wheel, or similar machine that drives an electric generator.

Prime Supplier: A firm that produces, imports, or transports selected petroleum products across State boundaries and local marketing areas, and sells the product to local distributors, local retailers, or end users.

Producing Region: In align with the American Gas Association definition, the Energy Information Administration adopted three natural gas storage regions: the Producing Region, the East Region, and the West Region. The Producing Region includes the following states: Alabama, Arkansas, Kansas, Louisiana, Mississippi, New Mexico, Oklahoma, and Texas. Also see map.

Production Capacity: The volume of product that can be generated by a production plant or enterprise in a given period by using current resources.

Programmable Controller: A device that controls the operation of electrical equipment (such as air conditioning units and lights) according to a preset time schedule.

Propane: A gas that is both present in natural gas and refined from crude oil. It is used for heating, lighting and industrial applications. The term propane also includes liquefied petroleum gases such as butane and ethane. See LPG.

Proven Reserves: The estimated quantity of crude oil or natural gas which geological and engineering data demonstrate with a given probability to be recoverable in the future assuming current costs of operation and market prices.

Provider Of Last Resort: A legal obligation (traditionally given to utilities) to provide service to a customer where competitors have decided they do not want that customer's business.

psi: Pounds per square inch. A unit of pressure, which is a unit measuring force per unit area.

Public Power: Consumer-owned electric utilities, either political subdivisions of the state such as public power districts and municipal systems, or cooperatives owned by their members.

The Public Utility Holding Company Act of 1935 (PUHCA): This Act prohibits acquisition of any wholesale or retail electric business through a holding company unless that business forms part of an integrated public utility system when combined with the utility's other electric business. The legislation also restricts ownership of an electric business by non–utility corporations.

The Public Utility Regulatory Policy Act of 1978 (PURPA): 1) Among other things, this federal legislation requires utilities to buy electric power from private "qualifying facilities," at an avoided cost rate. This avoided cost rate is equivalent to what it would have otherwise cost the utility to generate or purchase that power themselves. Utilities must further provide customers who choose to self–generate a reasonably priced back–up supply of electricity. 2) PURPA is implemented by the Federal Energy Regulatory Commission. Under PURPA each electric utility is required to offer to purchase available electric energy from cogeneration and small power production facilities.

Pumped Hydroelectric Storage: Commercial method used for large–scale storage of power. During off–peak times, excess power is used to pump water to a reservoir. During peak times, the reservoir releases water to operate hydroelectric generators.