Glossary of Solar Terms

Alternate fuels

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

Alternating Current (AC)

An electric current that alternates direction between positive and negative cycles, usually 50 or 60 times per second. Alternating current is the current typically available from power outlets in a household.


A measure of electrical current. It is the number of electrons flowing past a given point in an electrical conductor in a given amount of time.

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.

Base load

minimum energy level a company provides its customers on a constant basis.


Any organic material (especially trees and other vegetation) grown or produced for use as a renewable energy resource. Also called stored solar energy, wood burning, agricultural wastes and/or methane gases spin turbines that then generate electricity.


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


Metric thermal unit: the amount of heat energy needed to raise the temperature of one kilogram of water by one degree Centigrade. The word is used in two contexts: large calories and small calorie. The “small calorie” used in fuel research, is the amount of energy needed to raise the temperature of one gram of water one degree Celsius.

Carbon Dioxide (CO2)

The colorless, odorless gas that is formed during normal human breathing. It is also emitted by combustion activities used to produce electricity. CO2 is a major cause of the greenhouse effect that traps radiant energy near the earth’s surface.

Clean Energy

synonymous with renewable energy resources.


Usually a reflective mirror that focuses and enhances the sun’s rays onto a photovoltaic surface. This may sometimes refer to a lens with the same purpose.

Competitive Power Supply

Any company that (usually generates then) sells power.

Connection Charge

Fee demanded by utilities for the cost of hooking up new service.


Achieving the reduction of energy usage through increased efficiency or reduced waste.

Deep cycle battery

A battery designed to tolerate up to 80% discharge of its capacity.

Depth of discharge (DOD)

the amount of energy (expressed in percentage of its rated capacity) withdrawn from a battery, a battery bank or a cell.


Governmental relaxation of controls. In terms of the electricity market, deregulation refers to ending monopolies by local utilities and allowing privatization.

Direct Current (DC)

Electric current flowing in one direction in invariable amperage. This is most often associated with battery power.

Distributed Generation (Distributed Energy Resources)

Electricity that is provided by small power generators located at or near end users.

Electric utility

Any agency or authority aligned with distribution facilities for delivery of electrical energy to the public. The notion of utility varies widely from state to state and from time to time, depending on fickle laws concerning regulation and deregulation.

Electric Utility Restructuring

Also called deregulation, this is the introduction of competition into various phases of electricity production.


In the context of global warming, this is the release of radiatively relevant greenhouse gases (one example is the release of carbon dioxide during fuel combustion in an automobile engine.)

Energy Efficient

A qualification of electrical products, in which they achieve an appropriate ratio of energy usage to work-type output. For example, a light bulb will use most of the input electrical energy to produce light, not heat.


The capacity to do work.

Energy Sources

Three primary categories are fossil fuels (coal, oil, gas); nuclear (fission and fusion); and renewable (solar, wind, geothermal, biomass, hydro).

Escape Provision

A contract provision allowing a party, such as an electric customer, to get out of it, usually after paying a pre-set penalty.

Evacuated Tube

Used in solar thermal collectors, these are glass absorber tubes with the air evacuated and through which collector fluids (water or perhaps glycol) flows.


In the USA, The Federal Energy Regulatory Commission is an independent government agency that regulates rates charged by public utilities. Its jurisdiction is somewhat disputed but it includes the sale of electric power for resale.

Flat Plate Pumped

Most widely used among solar thermal collectors, this typically consists of a metal frame, glazing, absorbers (usually metal), and insulation with a pump liquid (typically glycol) as the heat-transfer medium: used in water heating applications.

Fluorescent light

A lighting device which uses an electrified gas rather than filament.

Fossil fuels

Fuels (oil, coal and gas) formed eons ago from decayed plants and animals.

Fuel Cell

A device producing electricity at high efficiency using a fuel and a chemical (usually an oxidizer) that reacts with it at two separate terminals, producing electricity.

Gaia Hypothesis

Theory that Earth is a self-perpetuating living system. Gaia is ancient Greek for “Mother Earth.”


Heat from the earth. Customarily applied to energy from geysers and hot springs (which is now better characterized as hydro-geothermal), in recent years, this term is applied to any heat stored in earth and available as a renewable energy resource.


The act of producing electricity.

Global warming

The earth’s gradual warming due to the “greenhouse effect.”

Greenhouse effect

The effect achieved by the buildup of gases (e.g. CFC’s and carbon dioxide) in the earth’s atmosphere causing the earth to become hot (as a greenhouse).


The electrical distribution system, as laid out by wires, conductors, stations, etc.

Grid-Interactive System.

See Utility interactive system. Also called a Grid-inter-tied system.

Heat Pump (Geothermal)

A heat pump in which a closed loop configuration circulates a fluid whose heat (or cooling) is exchanged to the heating (or ventilation) of a house.


Power obtained from the (typically gravitational) movement of water.

Incandescent light

A bulb using resistance to produce light when an electrical current passes through it. The conductor is usually a wire or filament.


Subsidies, tax breaks and other Government actions, in which indirect money is given to consumers under certain conditions.


Solar radiant energy impinging on the earth in any given region or area.


A device that changes direct current (DC) into alternating current (AC).


a measure of work or energy used in the Imperial System. measure of work or energy. A simple definition is that a joule is the work required to lift a mass of about 100 g for one meter under the earth’s gravity.

Kilowatt (kW)

One thousand watts of electricity (See Watt).

Kilowatthour (kWh)

One thousand watthours.

Lead-acid battery

Slightly larger than a car battery, these have internal plates made of lead or a lead alloy, surrounded by a sulfuric acid electrolyte. Car batteries are made with softer plates. Lead-acid batteries are commonly used in solar and wind energy systems.

Line wire loss

Voltage or power lost due to the resistance of wires in an electric circuit. Power loss is often proportional to wire length and is far greater in DC than AC.


the electricity required and used by any single or collection of electrical item(s).

Load Profile

Collected information on a customer’s usage over a set period of time. It is sometimes shown as a graph on the customer’s bill.

Maximum power point (MPP)

The voltage at which a PV array is producing maximum power.

Maximum power point tracker (MPPT)

A power conditioning unit that increases the power of a PV system by ensuring operation of the PV generator at its Maximum Power Point (MPP). The ability to do so can depend on climate and the battery’s state of charge.

Megawatt (MW)

One million watts of electricity (See Watt).


The absence of competition in an economic relationship. For example, many utilities have a monopoly on markets. Deregulation opens up the market to competition.

Net Metering

An arrangement permitting a household or business (using a utility meter that spins forward and backward, reading both inflows and outflows of electricity) to sell excesses of power generated over its load back to the utility supplier to offset costs.

Nuclear fusion

A potentially limitless source of energy in which nuclei are fused, with an accompanying release of energy.


Ocean Thermal Energy Conversion. Heat engines produce electrical power from the differential in temperature between the ocean’s warm surface and cold deep water.

Ocean energy

The use of the vast potential energy within the oceans. Several kinds are possible, including OTEC and wave power.

Off Peak/ On Peak

Times when energy demand and price is low (off-peak) or high (on-peak).


A major agent in the formation of air pollution, this is actually a form of oxygen. Ozone is a result of photochemical reactions involving emissions from automobile and industrial processes. Ozone does occur naturally in earth’s upper atmosphere (the ozone layer) where it protects the earth’s surface against harmful effects of the sun’s radiation. Ozone accumulation close to the earth’s surface is harmful. Ozone depletion in the upper atmosphere is also harmful, as the ozone layer is essential to the preservation of the earth.

Parabolic Dish

A high-temperature (above 180 degrees Fahrenheit) solar thermal concentrator, generally bowl-shaped, with two-axis tracking.

Parabolic Trough

A high-temperature (above 180 degrees Fahrenheit) solar thermal concentrator with the capacity for tracking the sun using one axis of rotation.

Passive Solar

A system in which solar energy alone is used for the transfer of thermal energy. Pumps, blowers, or other heat transfer devices that use energy other than solar are not used.

Peak load

The maximum demand for energy on a utility system which has to be met by the utility’s power generating capacity.

Peak Watt

A manufacturer’s unit indicating the amount of power a photovoltaic cell or module will produce at standard test conditions (normally 1,000 watts per square meter and 25 degrees Celsius).

Photovoltaic (PV)

the production of electricity from light. See also Solar cell.

Photovoltaic cell

see Solar cell.


The rate of doing work, expressed in watts (W). For example, a generator that is rated at 750 watts will be able to produce that amount of power continuously.

Public Utility Regulatory Policies Act (PURPA)

In the USA only: this is part of the National Energy Act. PURPA is intended to encourage the conservation of energy, more efficient use of resources, and equitable energy rates. Some important measures relevant to The Solar Guide include sections on net metering and incentives for renewable energy.

Pumped-storage hydroelectric plant

A plant that generates electric energy during peak load periods by using water previously pumped into an elevated storage reservoir during off-peak times. During peak times, water is released from the reservoir through a conduit to turbine generators located in a power plant at a lower level.


See Photovoltaic.

Quadrillion BTU

10 to the 15th power BTU.

Qualifying Facility

A small power production facility that meets ownership, operating, and efficiency criteria established under the Federal Energy Regulatory Commission (FERC) and the Public Utility Regulatory Policies Act (PURPA).


Governmental body’s rule concerning procedures that a utility must follow. As it concerns electricity, regulation is used to set rates, among other things.

Renewable energy

Energy from sources that cannot be used up or that are naturally replenishing: sunshine, water flow, wind and vegetation. Some add the qualification, “cannot be used up in one lifetime.” Still other note that these are “flow-limited,” in that there is a limited amount of energy per unit of time.

Renewable energy devices

Any device that harvests a renewable energy resource. Solar panels, solar collectors, wind machines, hydroelectric turbines, etc. are a few examples.

The Renewable Portfolio Standard (RPS)

In the USA, this is a mandate requiring that renewable energy provide a certain percentage of total energy generation or consumption.


Property of a material that resists the flow of electrons when a potential difference is applied across it. This is measured in Ohms. Resistance is the quality that causes electricity to produce light in a filament or heat in an electrical heater of any kind.


Any electronic component that restricts the flow of electrical current in circuits. These are often used intentionally to produce heat or light (e.g. a stove element or a light bulb filament).

Ribbon Silicon

A type of single-crystal silicon that produced in sheets or ribbons.


Semiconductor material made from silica, purified for photovoltaic applications.

Single Crystal Silicon

An very pure (and very expensive) form of crystalline silicon produced by the Czochralski method. This involves dipping a single crystal seed into a reservoir of molten silicon under refined conditions and slowly withdrawing a solidifying single crystal rod of silicon that is then fabricated into single-crystal photovoltaic cells.

Solar cell

A device made of silicon and semiconductor materials, producing electricity when exposed to sunlight. Movement of electrons is produced by fabrication of adjacent layers of materials with different characteristics.

Solar cooling

The use of devices that absorb sunlight to facilitate convection, similar to processes used in gas-fired refrigerators.

Solar electricity

Electricity produced in photovoltaic processes, by action of sunlight.

Solar energy

The radiant energy of the sun. This can be converted into usable forms of energy, such as heat or electricity.

Solar modules

Also called solar panels, these are the large collections of solar cells that can produce electricity at a worthwhile rate. These are packaged for end use and suited to be incorporated into solar energy or solar electricity) systems.

Solar panels

See solar modules.

Solar Photovoltaic

The process of converting the sun’s energy into electricity.

Solar Thermal collector

A device designed to collect solar radiation and convert it into thermal energy (usually to heat water).

Solar Thermal energy systems

Systems that collect the sun’s energy to produce heat, usually to heat water.

Stand-Alone system

A solar PV installation that is not connected to a utility power line (the grid).


Financial assistance by any government body that is granted directly to firms or individuals.


Pertaining to heat gain and heat retention.

Thermosiphon System

A type of solar thermal collector (with water heated directly) in which circulation through the storage loop is provided solely by the heating of the water.

Thin-Film Silicon

Most often this is amorphous (non-crystalline) material used to make photovoltaic (PV) cells.


in solar electric and solar thermal systems, a device used to track the sun.

Transmission System (Electric)

Lines or wires of electricity that transmit electrical energy from points of origin to end users.


Any machine used to generate rotary mechanical power from the energy of a stream of fluid (such as water, steam, or hot gas) or air (wind power). Turbines convert kinetic energy into mechanical energy.

Uninterruptible power supply

A device (likely containing batteries) that stores power for use when conventional power is unavailable (e.g. during a blackout).

Utility-Interactive System

Any solar PV installation that is connected to and interacting with a utility power line.


The unit of electric potential (intensity) and potential difference. It is also a relation of current to resistance: i.e. 1 volt is the potential difference across a resistance of 1 Ohm when a current of 1 Amp is flowing.

Voltage drop

The voltage lost along a length of wire or conductor due to resistance. The voltage drop is calculated using Ohm’s Law. Voltage drop is also measured for resistors.

Voltage regulator

A device to control the operating voltage of a photovoltaic array.


The measurement of the “intensity” of electricity.


Raw material for a solar cell, a thin sheet of crystalline semiconductor material is made by mechanically sawing it from a single-crystal boule or by casting it.

Watt (Electric)

The basic unit of electrical power. It is only ever defined in reference to current and intensity– as in 1 watt equals 1 ampere of current under pressure of 1 volt.

Watt (Thermal)

A unit of power in the metric system, expressed as energy per second.

Watthour (Wh)

A measure of energy, equal to 1 watt of power used or received by an electric circuit steadily for the duration of 1 hour.


The shape of a wave that represents a vibration or AC current.

Wind energy

Energy from wind motion converted into mechanical energy by the action of a wind turbine driving a generator with its central shaft.

Wind farms

see wind power plant below.

Wind turbines

Devices consisting of blades that turn a shaft that turns a generator to harvest wind energy and produce electrical power.

Wind power plant

Also called a wind farm, this is a group of turbines interconnected to a common utility system via a system of transformers, distribution lines, and a substation. This term is more common in the United States. Europeans call these generating stations.