Solar Glossary


The material in a solar panel that absorbs sunlight to capture solar radiation. Absorbers are typically made of metal and coated with materials that absorb and retain heat better than plain black paint.

Alternating Current

An electrical current that reverses direction periodically. You can measure alternating current (abbreviated as AC) in hertz, which is the number of complete cycles within one minute. In the United States, most electrical outlets are AC and complete 60 cycles per minute, which means they’re 60 hertz.

Ambient Temperature

The temperature of the air encompassing an environment or piece of equipment. In technical terms, it’s the temperature of the environment around a power supply or computer.


The unit used to measure electrical current. The International System of Units defines an ampere (abbreviated as amp) as equaling one coulomb per second.


The measurement of how much electricity can transfer through a current of one ampere in an hour, equal to 3600 coulombs. Abbreviated as AH or Ah.

Ampere Hour Meter

A device that measures the flow of current over time. An ampere-hour meter (or AH meter) is used to calculate electricity usage at residences and businesses. In solar charging systems, ampere hour meters are used to measure the batteries’ charging and discharging AH rate.

Annual Solar Savings

The energy savings caused by a solar energy system over the course of one year. The annual solar savings is calculated based on the energy used by a non-solar system. Your annual solar savings will vary depending on how much direct sunlight your house receives, and how much you’re currently paying for electricity. A solar system requires a large up-front investment, but you won’t need to pay any monthly fees as long as the power generated by your solar panels matches or exceeds the electricity you use each month.


A positively charged electrode that current flows through into a polarized electrical device. Because an anode has a positive charge, it attracts negatively charged electrons from within the device out to the outside circuit.

Antireflection Coating

A material that coats the surface of solar cells that reduces light reflection and increases light absorption. The antireflection coating on a solar cell will usually be silicon nitride or titanium oxide. Antireflection coating significantly increases the energy produced by a solar cell.


The optimal amount of time it takes for a photovoltaic (or solar energy) system to produce energy over a period of time, typically measured in hours per year. It’s also known as “uptime.”

Balance of System

Consists of all the components in a solar energy system except the solar panels themselves. The balance of system (or BOS) accounts for most of the installation and maintenance costs of a solar energy system and includes all the wiring, mounting hardware, batteries and battery chargers, as well as other components.


A portable device that provides power to electrical equipment. A battery consists of power cells that convert chemical energy into electric power. There are three main types of batteries used in photovoltaic systems—lead-acid (the least expensive but most short-lived option), lithium ion (the most commonly used and most efficient choice), and saltwater (a new, untested option that is better for the environment than other types of batteries).


A metallic element that is added to silicon to create p-type silicon, which you can use as a semiconductor in solar power cells. Since boron has one less electron than is needed to form a bond with silicon, it attracts electrons from neighboring semiconductors, creating an electric field.

British Thermal Unit

A unit used to measure heat. Specifically, a British thermal unit (or Btu) is that amount of heat necessary to raise the temperature of a pound of water by one degree Fahrenheit. Btu is an imperial unit and is the metric equivalent of calories. British thermal units are used to measure the potential energy production of a heating system.


The opposite of an anode. A cathode is a negatively charged electrode that current flows through into a polarized electrical device. Because a cathode has a negative charge, it attracts positively charged electrons from within the device out to the outside circuit. Though cathodes in solar systems are typically made of metal, researchers in India have found a way to construct them out of human hair.


A substance which allows the flow of energy or currents. An electrical conductor allows electrons to move across atoms when voltage is applied. Most metals are excellent conductors, but copper is the most common because of how relatively inexpensive it is compared to other materials.


An electronic device with an anode and a cathode, typically made from semi-conductive materials. A diode’s primary purpose is to conduct electricity in only one direction.

Direct Beam Radiation

Solar radiation that travels directly from the sun to the earth’s surface without any interruption or deflection. Clouds and pollution can cause solar radiation to become more diffuse, and the angle of the sun also affects how directly solar radiation hits the surface.

Direct Current

An electrical current that can only travel in one direction. Many electronic devices use DC, while most electrical outlets in homes and businesses use AC; thus, the AC adapter bricks that are attached to some electronics’ power cords.


A release of stored electricity from a battery. Cold weather cause batteries to discharge faster, because the chemical reactions within batteries are slowed down by cold temperatures. Batteries also discharge faster when they have to work harder, such as during peak usage times in very hot or very cold weather.

Dry Cell

A battery that uses a dry paste material to conduct electricity rather than a liquid conductor. Alkaline and lithium batteries are dry cell batteries. Unlike wet cell batteries, dry cell batteries can work in any orientation because the conductor doesn’t shift around during movement.

Electric Circuit

A passage through which an electric current travels through conductors when voltage is applied. Voltage provides the force that pushes electrons through the circuit, and various transistors and capacitors make up the load that the current must pass through.

Electric Current

A stream of electricity flowing through conductors. You can measure current in amperes (or amps). In an electric circuit, the current is made up of electrons moving through a wire.


A component of a battery or cell that conducts electricity to or from a non-metallic object, such as a vacuum or semiconductor. An electrode can be either an anode or a cathode.

Energy Audit

An assessment of how much energy a home or business is using, along with recommendations for how to be more energy efficient. Professionals can perform energy audits, or you can perform one on your own with instructions from the US Department of Energy. An energy audit requires investigating each room in a home or business to make a note of how many electronic devices and electrical outlets are in use.

Fixed Tilt Array

An array of solar panels that are placed at fixed angles in order to capture as much sunlight as possible. Fixed tilt arrays are the easiest to design, install, and maintain because they don’t move once they’re in place.

Flat-Plate Array

An array of flat solar panels that use both direct and diffuse solar radiation. Flat-plate arrays can be fixed in place or can be moved to track the movement of the sun.

Flat-Plate Module

A single solar panel in a flat-plate array. Each module consists of cells arranged on a flat surface and exposed to incoming sunlight.


The rate at which something repeatedly occurs over a period of time. For example, frequency is used to describe the number of complete cycles of alternating current, which you measure in Hertz.

Gel-type Battery

A lead acid battery that uses the chemical reaction of an electrolyte combined with silica fumes. The reaction creates a gel-like material, which is where the battery gets its name. Gel-type batteries produce very little gas or fumes, which makes them safe to use in areas with poor ventilation.


A unit used to measure electrical power. A gig is one billion, so a gigawatt equals one billion watts. Thus, gigawatts (abbreviated as GWs) are used to measure high-energy systems.

Hybrid System

An energy system that uses multiple sources of electricity generation. In solar energy, a hybrid system refers to a system that combines solar power with wind, hydroelectric, or fossil fuel generators. These are common in homes and offices that use more electricity than a solar system can provide.


An electronic gadget that converts direct current (DC) electricity into alternating current (AC) electricity. An inverter can be used to power household electronics and appliances, which typically use AC electricity, with a DC battery.


A unit for measuring energy production. A joule is equal to the energy required to use one newton of force over a distance of one meter. A Newton (abbreviated as N) is defined as the amount of force required to move one kilogram of mass at a rate of one meter per second squared, and can be represented by the formula F=ma where “F” is force, “m” is mass, and “a” is acceleration.


A unit for measuring electrical power. Smaller than a gigawatt, a kilowatt (abbreviated as kW) is equal to one thousand watts.


A measure of energy, in units of kilowatts, multiplied by a period of time in hours. Utility companies use kilowatts in billing for electricity consumption in homes and businesses. Kilowatt-hours can be abbreviated as kWh.


A measurement of energy dispersion over a specified area. A Langley (abbreviated as Ly) is used to measure solar energy and is equal to one thermochemical calorie per square centimeter.

Lead-Acid Battery

A wet cell battery that you can recharge. It generates electrical current by using lead oxide and sulfuric acid to oxidize a lead plate. Lead-acid batteries are currently the cheapest form of rechargeable battery, but they contain toxic lead and acid. They typically don’t last as long as other types of batteries.

Light Trapping

A method of trapping light in a solar cell by giving it a textured surface and angling it in a way to capture the most solar radiation. This method reduces reflection and increases absorption rates.


A component in an electrical circuit that draws or consumes power, rather than generating or conducting it. A load will typically transform electrical current into other forms of energy like heat or light. Appliances and lights are examples of electrical loads.


Measurement of electric energy that is equal to one million watts. The prefix “mega” means million. In practical terms, one megawatt (abbreviated as MW) is roughly the amount of power generated by ten car engines.


A measure of energy usage over a time period represented in hours. One megawatt-hour is equivalent to one thousand kilowatt-hours, or one million watt-hours. Megawatt-hour is abbreviated as Mwh.

National Electrical Code

A series of guidelines for installing electrical wiring and equipment. The National Electrical Code (abbreviated as NEC) contains information on how to safely install and use photovoltaic (or solar) energy systems.

National Electrical Manufacturers Association

A national trade group made up of electrical equipment manufacturers. The National Electrical Manufacturers Association (NEMA) publishes standards, guides, and other papers on the generation, control, and use of electric power.

Nickel-Cadmium Battery

A rechargeable battery with electrodes made from nickel and cadmium. Nickel-cadmium batteries (abbreviated as NiCds or NiCads) are used in small devices like laptops and cameras.


A unit for measuring electrical resistance in a circuit. One ohm is equivalent to the resistance of a circuit or conductor when one volt is applied and maintains a current of one ampere.

Peak Demand/Load

The highest electricity demand of a system over a specified time period. Peak demand or peak load (which you can use interchangeably) are typically used to anticipate future electricity needs for a customer or region during high-usage times. Extreme temperatures in the summer and winter are usually expected to be peak demand or peak load times.

Photoelectric Cell

A device that converts radiation from the sun into electrical current or voltage. Commonly referred to as a photovoltaic cell or a solar cell. Most photoelectric cells are made of thin-film crystalline silicon.

Photoelectrochemical Cell

A type of solar cell that irradiates an anode with solar radiation and electrolyzes water to create hydrogen and oxygen gas. This process allows the cell to store the energy from incoming sunlight for future use. Photoelectrochemical cells are usually made of semi-conductive metal oxides like TiO2 (titanium dioxide).


A material that is capable of conducting an electric current only under certain conditions. A semiconductor is halfway between a conductor and an insulator and must heat up to conduct electricity. Commonly used semiconductor materials are metal oxides and silicons that have been specially prepared for this purpose.

Soft Costs

The non-hardware costs involved in the installation and maintenance of a photovoltaic or solar energy system. Soft costs typically include labor, finance charges, permit fees, and inspections. Soft costs usually make up about 64% of the total price of a solar energy system.

Solar Energy

The energy that is created by the sun and radiated down to the earth in the form of sunlight and heat. This radiation can be harnessed with photovoltaic or solar systems and transformed into electricity. Solar energy is a much more environmentally-friendly method of generating electricity than traditional natural gas and coal.


A group of solar modules that have been interconnected to form a circuit. A photovoltaic string is made up of the specific number of modules or panels required to produce a particular amount of energy.

Thermophotovoltaic Cell

A type of solar cell that absorbs solar radiation in a process that raises its temperature to 1,000-1,200 degrees Celsius. That heat is then used to produce photons which generate electricity.

Tilt Angle

The angle, relative to the horizontal plane, at which you can tilt a solar panel to capture the most solar radiation from the sun.


An electrical device that is used to increase or decrease the voltage of an alternating current. A transformer transfers energy between multiple circuits through electromagnetic induction, which is the process of sending voltage across circuits in a variable magnetic field.


A unit that measures the strength of an electrical current. You can measure a volt (V) as the difference of potential between two points with a current of one ampere when the power usage equals one watt.


The actual measurement of the strength of an electrical current. Volts are the unit of measurement for voltage.


A unit of measurement of power (energy over time). Watts are used to calculate the rate of energy transfer, with one watt equaling one joule per second. Watt is abbreviated as W.

Zenith Angle

The angle between the zenith (or the vertical point that, if you drew a line, would create a 90-degree angle with the ground) and the center of the sun. The solar zenith angle is used to calculate the optimal position and tilt of solar panels.

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