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Alternate fuels - Any fuel that is not considered mainstream but may prove just as useful.

Alternating Current (AC) – The type of electrical current where the direction of flow is reversed at regular intervals or cycles.  This is the standard type of utility-provided current in the United States which cycles 60 times per second.

Amorphous Silicon - A form of silicon that is noncrystalline. Also called thin film, it is different from regular silicon used in panels and chips, in that it is more flexible. It can be deposited in thin (measured in micrometers) layers by a number of methods.

Amp or Ampere - The amp is a measure of the amount of electric charge passing a point in an electric circuit per unit of time.  Amps can be viewed as the flow rate.

Array – A number of solar photovoltaic (PV) panels wired together to function as a unit.

Biomass - Any organic material (especially trees and other vegetation) grown or produced for use as a renewable energy resource. Sometimes referred to as stored solar energy.

BTU - British Thermal Unit-the amount of heat energy needed to raise the temperature of one pound of water by one degree Fahrenheit.

Concentrator - Typically a reflective mirror that focuses and enhances the sun's rays onto a photovoltaic surface or a solar thermal collector. This may sometimes refer to a lens with the same purpose.

Depth of Discharge (DOD) - The amount of energy withdrawn from a battery, or a battery bank expressed as a percentage of its rated capacity.

Direct Current (DC) – The type of electrical current with the flow always going in one direction.  This is the type of current produced by many renewable energy sources and must be converted to AC to be compatible with standard household appliances.

Distributed Generation – The term for localized or on-site electrical generation, as opposed to central generation at a commercial power plant.

Evacuated Tube - A type of solar thermal collector which utilizes a heat pipe inside a vacuum tube to absorb energy from the sun.  This heat is then transferred from the hot end of the vacuum tube/heat pipe to transfer fluid (typically propylene glycol) in a manifold.

Flat plate collector -This is the most popular style of residential solar thermal collector.  It consists of a metal-framed insulated box with a transparent cover that allows solar energy to pass through.  Inside, a dark flat-plate absorber collects solar energy and transfers it to a heat-transfer fluid (typically propylene glycol) which is circulated through the collector.

Fluorescent light - A lighting device which uses an electrified gas rather than a filament.

Geothermal - Applies to any heat stored in the earth and available as a renewable energy resource.

Grid - An integrated system for the distribution of electricity.  It includes the generating facilities, and all the power poles, wires, transformers, etc. to distribute the power over a wide area.

Grid-connected System – A renewable energy system that is connected to the community electrical grid.  See “grid” above.

Insolation - Solar radiant energy striking the earth in a given area.  Also called irradiance, and is typically stated as kilowatt-hours per square meter per day (kWh/m2/day).

Inverter – A device that converts direct current (DC) electricity into alternating current (AC) to be used on-site or to interact with the grid.

Kilowatt (kW) - The standard unit of electrical power equal to 1,000 watts, or to energy consumption at a rate of 1,000 joules per second.  A 100 watt light bulb consumes 100 watts of power when it is on.  If left on continuously for 10 hours, the bulb would use 1,000 watt-hours or one kilowatt-hour of electricity.

Kilowatt-Hour (kWh) – A kWh is a unit of energy equal to 1,000 watts acting over a period of one hour.

Line Loss or Line Wire Loss - Voltage or power lost due to the resistance of wires in an electric circuit. Power loss increases proportional to increasing wire length.

Load – The energy consumption or requirement of a piece of equipment or collection of equipment that places a demand on a specific portion of the electrical system.

Maximum Power Point (MPP) - The operation point at which a PV array is producing maximum power.

Maximum Power Point Tracker (MPPT) - A power conditioning unit that increases the performance of a PV system by focusing operation of the system to its Maximum Power Point (MPP).

Net Metering - Refers to policy that requires utility companies to purchase power from small-scale residential customers/suppliers.  Purchase rates are not required to be at full retail price, and are frequently at a greatly reduced "wholesale" rate.

Passive Solar - A system or design in which solar energy is collected directly and utilized for its thermal energy without the use of any mechanical devices whatsoever.

Photovoltaic (PV) – The effect of converting light directly into electricity.

Photovoltaic Panel or Module – A commercially manufactured unit comprised of interconnected PV cells used to generate DC electricity.  They are typically rectangular in shape, and 2 or 3 feet by 3 or 4 feet in size.

Renewable Energy - Sources of energy that are replenished or recurring naturally; such as, sunshine, wind, flowing water and biomass/vegetation.

Solar Energy - Radiant energy from the sun which can be used directly as "passive solar" heat, or used to generate electricity in a "photovoltaic panel," or used to heat water or a heat-transfer fluid in a "solar thermal panel."

Solar Thermal Panel - A device to collect solar radiant energy and use it to heat a fluid.  Typically a thin rectangular box with piping inside that heats a heat-transfer fluid which then heats domestic hot water or a space heating system.

Thermal - Pertains to temperature or heat.

Tracker - A style of mounting a solar electric array on a rack with a mechanical device which moves the array to keep it pointed toward the sun.  The array "tracks" the path of the sun to maximize the electric output of the array.

Watt (W) – The basic unit of electrical power, measured at any moment.