Packing Factor
The ratio of solar collector array area to actual land area.

Pane (Window)

The area of glass that fits in the window frame.

Panel (Solar)

A term generally applied to individual solar collectors, and typically to solar photovoltaic collectors or modules.

Panel Radiator

A mainly flat surface for transmitting radiant energy.


A drag-type wind machine that can react to wind from any direction.

Parabolic Aluminized Reflector Lamp

A type of lamp having a lens of heavy durable glass that focuses the light. They have longer lifetimes with less lumen depreciation than standard incandescent lamps.

Parabolic Dish

A solar energy conversion device that has a bowl shaped dish covered with a highly reflective surface that tracks the sun and concentrates sunlight on a fixed absorber, thereby achieving high temperatures, for process heating or to operate a heat (Stirling) engine to produce power or electricity.

Parabolic Trough

A solar energy conversion device that uses a trough covered with a highly reflective surface to focus sunlight onto a linear absorber containing a working fluid that can be used for medium temperature space or process heat or to operate a steam turbine for power or electricity generation.


A configuration of an electrical circuit in which the voltage is the same across the terminals. The positive reference direction for each resistor current is down through the resistor with the same voltage across each resistor.


The fine liquid or solid particles contained in combustion gases. The quantity and size of particulates emitted by cars, power and industrial plants, wood stoves, etc are regulated by the U.S. Environmental Protection Agency.

Parallel Connection

A way of joining photovoltaic cells or modules by connecting positive leads together and negative leads together; such a configuration increases the current, but not the voltage.


A chemical reaction that eliminates the detrimental effect of electrically reactive atoms on a photovoltaic cell’s surface.

Passive/Natural Cooling

To allow or augment the natural movement of cooler air from exterior, shaded areas of a building through or around a building.

Passive Solar (Building) Design

A building design that uses structural elements of a building to heat and cool a building, without the use of mechanical equipment, which requires careful consideration of the local climate and solar energy resource, building orientation, and landscape features, to name a few. The principal elements include proper building orientation, proper window sizing and placement and design of window overhangs to reduce summer heat gain and ensure winter heat gain, and proper sizing of thermal energy storage mass (for example a Trombe wall or masonry tiles). The heat is distributed primarily by natural convection and radiation, though fans can also be used to circulate room air or ensure proper ventilation.

Passive Solar Heater

A solar water or space-heating system in which solar energy is collected, and/or moved by natural convection without using pumps or fans. Passive systems are typically integral collector/storage (ICS; or batch collectors) or thermosyphon systems. The major advantage of these systems is that they do not use controls, pumps, sensors, or other mechanical parts, so little or no maintenance is required over the lifetime of the system.

Passive Solar Home

A house built using passive solar design techniques.

Payback Period

The amount of time required before the savings resulting from your system equal the system cost.

Peak Clipping/Shaving

The process of implementing measures to reduce peak power demands on a system.

Peak Demand/Load

The maximum energy demand or load in a specified time period.

Peaking Capacity

Power generation equipment or system capacity to meet peak power demands.

Peaking Hydropower

A hydropower plant that is operated at maximum allowable capacity for part of the day and is either shut down for the remainder of the time or operated at minimal capacity level.

Peak Power

Power generated that operates at a very low capacity factor; generally used to meet short-lived and variable high demand periods.

Peak Shifting

The process of moving existing loads to off-peak periods.

Peak Sun Hours

The equivalent number of hours per day when solar irradiance averages 1 kW/m2. For example, six peak sun hours means that the energy received during total daylight hours equals the energy that would have been received had the irradiance for six hours been 1 kW/m2.

Peak Watt

A unit used to rate the performance of a solar photovoltaic (PV) cells, modules, or arrays; the maximum nominal output of a PV device, in Watts (Wp) under standardized test conditions, usually 1000 Watts per square meter of sunlight with other conditions, such as temperature specified.

Peak Wind Speed

The maximum instantaneous wind speed (or velocity) that occurs within a specific period of time or interval.


Solid fuels made from primarily wood sawdust that is compacted under high pressure to form small (about the size of rabbit feed) pellets for use in a pellet stove.

Pellet Stove

A space heating device that burns pellets; are more efficient, clean burning, and easier to operate relative to conventional cord wood burning appliances.

Pelton Turbine

A type of impulse hydropower turbine where water passes through nozzles and strikes cups arranged on the periphery of a runner, or wheel, which causes the runner to rotate, producing mechanical energy. The runner is fixed on a shaft, and the rotational motion of the turbine is transmitted by the shaft to a generator. Generally used for high head, low flow applications.


A component of a hydropower plant; a pipe that delivers water to the turbine.

Perfluorocarbon Tracer Gas Technique (PFT)

An air infiltration measurement technique developed by the Brookhaven National Laboratory to measure changes over time (one week to five months) when determining a building’s air infiltration rate. This test cannot locate exact points of infiltration, but it does reveal long-term infiltration problems.

Performance Ratings

Solar collector thermal performance ratings based on collector efficiencies, usually expressed in Btu per hour for solar collectors under standard test or operating conditions for solar radiation intensity, inlet working fluid temperatures, and ambient temperatures.

Perimeter Heating

A term applied to warm-air heating systems that deliver heated air to rooms by means of registers or baseboards located along exterior walls.


A unit of measurement for the ability of a material to retard the diffusion of water vapor at 73.4 F (23 C). A perm, short for permeance, is the number of grains of water vapor that pass through a square foot of material per hour at a differential vapor pressure equal to one inch of mercury.

Phantom Load

Any appliance that consumes power even when it is turned off. Examples of phantom loads include appliances with electronic clocks or timers, appliances with remote controls, and appliances with wall cubes (a small box that plugs into an AC outlet to power appliances).


Alternating current is carried by conductors and a ground to residential, commercial, or industrial consumers. The waveform of the phase power appears as a single continuous sine wave at the system frequency whose amplitude is the rated voltage of the power.

Phase Change

The process of changing from one physical state (solid, liquid, or gas) to another, with a necessary or coincidental input or release of energy.

Phase-Change Material

A material that can be used to store thermal energy as latent heat. Various types of materials have been and are being investigated such as inorganic salts, eutectic compounds, and paraffins, for a variety of applications, including solar energy storage (solar energy heats and melts the material during the day and at night it releases the stored heat and reverts to a solid state).

Photobiological Hydrogen Production

A hydrogen production process that process uses algae. Under certain conditions, the pigments in certain types of algae absorb solar energy. An enzyme in the cell acts as a catalyst to split water molecules. Some of the bacteria produces hydrogen after they grow on a substrate.


An electric current induced by radiant energy.

Photoelectric Cell

A device for measuring light intensity that works by converting light falling on, or reach it, to electricity, and then measuring the current; used in photometers.

Photoelectrochemical Cell

A type of photovoltaic device in which the electricity induced in the cell is used immediately within the cell to produce a chemical, such as hydrogen, which can then be withdrawn for use.

Photoelectrolysis Hydrogen Production

The production of hydrogen using a photoelectrochemical cell.

Photogalvanic Processes

The production of electrical current from light.


A particle of light that acts as an individual unit of energy.

Photovoltaic (Conversion) Efficiency

The ratio of the electric power produced by a photovoltaic device to the power of the sunlight incident on the device.

Photovoltaic (PV; Solar) Array

A group of solar photovoltaic modules connected together.

Photovoltaic (Solar) Cell

Treated semiconductor material that converts solar irradiance to electricity.

Photovoltaic Device

A solid-state electrical device that converts light directly into direct current electricity of voltage-current characteristics that are a function of the characteristics of the light source and the materials in and design of the device. Solar photovoltaic devices are made of various semi-conductor materials including silicon, cadmium sulfide, cadmium telluride, and gallium arsenide, and in single crystalline, multi-crystalline, or amorphous forms.

Photovoltaic (Solar) Module or Panel

A solar photovoltaic product that generally consists of groups of PV cells electrically connected together to produce a specified power output under standard test conditions, mounted on a substrate, sealed with an encapsulant, and covered with a protective glazing. Maybe further mounted on an aluminum frame. A junction box, on the back or underside of the module is used to allow for connecting the module circuit conductors to external conductors.

Photovoltaic Peak Watt

see Peak Watt.

Photovoltaic (Solar) System

A complete PV power system composed of the module (or array), and balance-of-system (BOS) components including the array supports, electrical conductors/wiring, fuses, safety disconnects, and grounds, charge controllers, inverters, battery storage, etc.

Photovoltaic-Thermal (PV/T) Systems

A solar energy system that produces electricity with a PV module, and collects thermal energy from the module for heating. There are no commercially available systems available (as of 11/97).

Physical Vapor Deposition

A method of depositing thin semiconductor photovoltaic) films. With this method, physical processes, such as thermal evaporation or bombardment of ions, are used to deposit elemental semiconductor material on a substrate.

Pitch Control

A method of controlling a wind turbine’s speed by varying the orientation, or pitch, of the blades, and thereby altering its aerodynamics and efficiency.


A semiconductor (photovoltaic) device structure that layers an intrinsic semiconductor between a p-type semiconductor and an n-type semiconductor; this structure is most often used with amorphous silicon PV devices.


A semiconductor (photovoltaic) device structure in which the junction is formed between a p-type layer and an n-type layer.


The space between a hanging ceiling and the floor above or roof; usually contains HVAC ducts, electrical wiring, fire suppression system piping, etc.

Plug Flow Digester

A type of anaerobic digester that has a horizontal tank in which a constant volume of material is added and forces material in the tank to move through the tank and be digested.

Point-Contact Cell

A high efficiency silicon photovoltaic concentrator cell that employs light trapping techniques and point-diffused contacts on the rear surface for current collection.


A semiconductor (photovoltaic) material composed of variously oriented, small, individual crystals.


A registered trademark for plastic sheeting material that can be used as a vapor retarder. This plastic is used to make grocery bags. It is a long chain of carbon atoms with 2 hydrogen atoms attached to each carbon atom.


(See Foam Insulation)

Porous Media

A solid that contains pores; normally, it refers to interconnected pores that can transmit the flow of fluids. (The term refers to the aquifer geology when discussing sites for CAES.)

Portfolio Standard

The requirement that an electric power provider generate or purchase a specified percentage of the power it supplies/sells from renewable energy resources, and thereby guarantee a market for electricity generated from renewable energy resources.

Potable Water

Water that is suitable for drinking, as defined by local health officials.

Potential Energy

Energy available due to position.

Pound of Steam

One pound of water in vapor phase; is NOT steam pressure, which is expressed as pounds per square inch (psi).

Pound Per Square Inch Absolute (psia)

A unit of pressure [hydraulic (liquid) or pneumatic (gas)] that does not include atmospheric pressure.


Energy that is capable or available for doing work; the time rate at which work is performed, measured in horsepower, Watts, or Btu per hour. Electric power is the product of electric current and electromotive force.

Power Coefficient

The ratio of power produced by a wind energy conversion device to the power in a reference area of the free windstream.

Power Conditioning

The process of modifying the characteristics of electrical power (for e.g., inverting dc to ac).

Power (Output) Curve

A plot of a wind energy conversion device’s power output versus wind speed.

Power Density

The amount of power per unit area of a free windstream.

Power Factor (PF)

The ratio of actual power being used in a circuit, expressed in watts or kilowatts, to the power that is apparently being drawn from a power source, expressed in volt-amperes or kilovolt-amperes.

Power Generation Mix

The proportion of electricity distributed by a power provider that is generated from available sources such as coal, natural gas, petroleum, nuclear, hydropower, wind, or geothermal.

Power Provider

A company or other organizational unit that sells and distributes electrical power (e.g., private or public electrical utility), either to other distribution and wholesale businesses or to end-users. Sometimes power providers also generate the power they sell.

Power (Solar) Tower

A term used to describe solar thermal, central receiver, power systems, where an array of reflectors focus sunlight onto a central receiver and absorber mounted on a tower.

Power Transmission Line

An electrical conductor/cable that carries electricity from a generator to other locations for distribution.

Preheater (Solar)

A solar heating system that preheats water or air that is then heated more by another heating appliance.

Present Value

The amount of money required to secure a specified cash flow at a future date at a specified return.

Pressure Drop

The loss in static pressure of a fluid (liquid or gas) in a system due to friction from obstructions in pipes, from valves, fittings, regulators, burners, etc, or by a breech or rupture of the system.

Pressurization Testing

A technique used by energy auditors, using a blower door, to locate areas of air infiltration by exaggerating the defects in the building shell. This test only measures air infiltration at the time of the test. It does not take into account changes in atmospheric pressure, weather, wind velocity, or any activities the occupants conduct that may affect air infiltration rates over a period of time.

Primary Air

The air that is supplied to the combustion chamber of a furnace.

Prime Mover

Any machine capable of producing power to do work.

Process Heat

Thermal energy that is used in agricultural and industrial operations.

Products of Combustion

The elements and compounds that result from the combustion of a fuel.

Producer Gas

Low or medium Btu content gas, composed mainly of carbon monoxide, nitrogen(2), and hydrogen(2) made by the gasification of wood or coal.

Programmable Thermostat

A type of thermostat that allows the user to program into the devices’ memory a pre-set schedule of times (when certain temperatures occur) to turn on HVAC equipment.

Projected Area

The net south-facing glazing area projected on a vertical plane. Also, the solid area covered at any instant by a wind turbine’s blades from the perspective of the direction of the windstream (as opposed to the swept area).


A hydrocarbon gas, C3H8, occurring in crude oil, natural gas, and refinery cracking gas. It is used as a fuel, a solvent, and a refrigerant. Propane liquefies under pressure and is the major component of liquefied petroleum gas (LPG).

Propeller (Hydro) Turbine

A turbine that has a runner with attached blades similar to a propeller used to drive a ship. As water passes over the curved propeller blades, it causes rotation of the shaft.

Proximate Analysis

A commonly used analysis for reporting fuel properties; may be on a dry (moisture free) basis, as “fired”, or on an ash and moisture free basis. Fractions usually reported include: volatile matter, fixed carbon, moisture, ash, and heating value (higher heating value).


Pounds of pressure per square inch.


Pounds/force per square inch absolute.


Pounds/force per square inch gauge.


An instrument for measuring relative humidity by means of wet and dry-bulb temperatures.


The analysis of atmospheric conditions, particularly moisture in the air.

P-Type Semiconductor

A semiconductor in which holes carry the current; produced by doping an intrinsic semiconductor with an electron acceptor impurity (e.g., boron in silicon).

Public Utility Holding Company Act (PUHCA) of 1935

A law to protect consumers and investors. It placed geographic restrictions on mergers and limitations on diversification into non-utility lines of business and takeovers of electric and gas utilities, and also established regulated monopoly markets or service territories for utilities.

Public Utilities Regulatory Policy Act (PURPA) of 1978

A law that requires electric utilities to purchase electricity produced from qualifying power producers that use renewable energy resources or are cogenerators. Power providers are required to purchase power at a rate equal to the avoided cost of generating the power themselves. (See Avoided Costs and Qualifying Facility)

Public Utility or Services Commissions (PUC or PSC)

These are state government agencies responsible for the regulation of public utilities within a state or region. A state legislature oversees the PUC by reviewing changes to power generator laws, rules and regulations and approving the PUC’s budget. The commission usually has five Commissioners appointed by the Governor or legislature. PUCs typically regulate: electric, natural gas, water, sewer, telephone services, trucks, buses, and taxicabs within the commission’s operating region. The PUC tries to balance the interests of consumers, environmentalists, utilities, and stockholders. The PUC makes sure a region’s citizens are supplied with adequate, safe power provider service at reasonable rates.

Pulse-Width-Modulated (PWM) Wave Inverter

A type of power inverter that produce a high quality (nearly sinusoidal) voltage, at minimum current harmonics.

Pumped Storage Facility

A type of power generating facility that pumps water to a storage reservoir during off-peak periods, and uses the stored water (by allowing it to fall through a hydro turbine) to generate power during peak periods. The pumping energy is typically supplied by lower cost base power capacity, and the peaking power capacity is of greater value, even though there is a net loss of power in the process.


A device used to measure total incident solar radiation (direct beam, diffuse, and reflected radiation) per unit time per unit area.


A device that measures the intensity of direct beam solar radiation.


The transformation on a compound or material into one or more substances by heat alone (without oxidation). Often called destructive distillation. Pyrolysis of biomass is the thermal degradation of the material in the absence of reacting gases, and occurs prior to or simultaneously with gasification reactions in a gasifier. Pyrolysis products consist of gases, liquids, and char generally. The liquid fraction of pyrolisized biomass consists of an insoluble viscous tar, and pyroligneous acids (acetic acid, methanol, acetone, esters, aldehydes, and furfural). The distribution of pyrolysis products varies depending on the feedstock composition, heating rate, temperature, and pressure.