Glossary Of Terms

Solar Terms and What They Mean

The content presented within Solar Topps’ Frequently Asked Questions, is meant solely for informative purposes and neither the individual contributors, developers, nor sponsors of Solar Topps, nor anyone else connected or affiliated with Solar Topps can take any responsibility for the results or consequences of any attempt to use or adopt any of the information presented.


Active Solar Thermal Technologies, which use fans or pumps to convert solar energy into usable heat or cooling (thermal energy). Active solar technologies use electrical or mechanical equipment, such as pumps and fans, to move the heat transfer fluid, air, water, anti-freeze, etc. from the surface where it is collected to the location where it is used to heat, cool, or dry something else.

Air Mass A measure of how far light travels through the Earth’s atmosphere. One air mass, or AM1, is the thickness of the Earth’s atmosphere. Air mass zero (AM0) describes solar irradiance in space, where it is unaffected by the atmosphere. The power density of AM1.5 light is about 1,000 W/m”; the power density of AM0 light about 1,360 W/m”.

Ampere A unit of electrical current. A potential of one volt across a resistance of one ohm causes a current of one ampere (6.25 X 1018 electrons per second) to flow.

Ampere-hour A unit of energy, typically referring to battery capacity. One ampere of current flowing for one hour.

Angle of incidence The angle between a ray of sunlight striking a surface and a line perpendicular to that surface. Rays perpendicular to a surface have a zero angle of incidence.

Array A number of solar modules electrically connected to produce a single electrical output.
Azimuth The angular measure between due south and the point on the horizon directly below the sun.

Balance of System (BOS) All the non solar collector components needed to harness the solar energy that falls on the collector surface. For a solar air system this usually includes a fan, ducts, and a simple thermostat. For water based heating systems this usually includes a pump, piping, antifreeze, a storage tank, thermostat and heat exchangers to transfer the heat to domestic water or space heat. For a solar electric system it usually includes wiring, an inverter to convert direct current to alternating current, electrical switch gear to connect to the existing electric grid or building power panels or both, and sometimes batteries and battery charging and monitoring equipment to store electricity.

Battery Two or more electrochemical cells connected to provide energy storage. Also commonly used to designate one cell.

Blocking diode A diode which prevents reverse current flow in a circuit, commonly used to prevent a battery from discharging through the array at night.

Btu British thermal unit, a measure of heat. One Btu is the amount of heat energy needed to raise the temperature of one pound of water by one degree F. This is the standard measurement used to state the amount of energy that a fuel gives off when burned as well as the amount of output of any heat generating device. You might be able to imagine it this way. Take one gallon (~8 pounds) of water and put it on your stove. Assume that the water starts at 60 degrees F and you want to bring it to a boil (212 degrees F.). To reach the boiling point, you will need about 1,200 Btu’s. Here’s the math: You need to raise each pound of water 152 degrees F. (212-60 = 152) and you have to raise temperature for 8 pounds, requiring 1,216 Btu’s (152 x 8 = 1216). To give some examples, It takes about 50 BTU to take a cup of cold tap water to hot coffee temperature. It takes about 600 BTU to warm a gallon of cold water to hot water for sinks and showers. A typical home furnace in the central United States produces about 100,000 BTU in one hour. One square foot of well oriented solar roof (or wall, or collector) can produce about 100,000 BTU in one winter and 200,000 BTU in one year.

Charge controller The PV system component which controls the battery’s state of charge. It may also provide other system control functions.

Charge rate The current applied to a battery to restore its energy capacity. The rate is typically normalized with respect to the battery’s full capacity and a designated time period. Thus, the current necessary to nominally charge a 100-ampere-hour battery from zero to full charge in five hours (20 amperes) is referred to as the battery’s C/5 rate. The term is also applied to discharge rate.

Concentrated Solar Thermal Power Systems Technologies using the sun’s heat to produce electricity. These systems typically concentrate the sun’s heat to boil water or another liquid at high temperatures. The steam from the boiling water rotates a large turbine, which spins a generator that produces electricity.

Concentrator array A photovoltaic (PV) array which uses concentrating devices (reflectors, lenses) to increase the intensity of the sunlight striking the array.

Czochralsky (CZ) method The method used to make monocrystalline silicon.

DC – direct current The type of current provided by a battery or solar cell, which flows in one direction.

Depth of discharge A measure of how much energy has been withdrawn from a battery, expressed as a percentage of full capacity. A 100 Ah battery from which 30 Ah has been withdrawn has undergone a 30% depth of discharge (DOD). This term is the inverse of state of charge (SOC); the example battery would be at 70% SOC.

Diffuse radiation The sunlight received indirectly, as a result of scattering due to clouds, fog, dust, moisture vapour or other substances in the atmosphere.

Diffusion The wafers are given a negative characteristic by exposure to a phosphorus source at high temperature.

Direct radiation Sunlight received directly, which has travelled in a straight path from the sun, also referred to as beam radiation.

Efficiency With respect to solar cells, the percentage of light energy that is converted to electricity by the cell. Depending on cell technology and production technique, this ranges from as low as 5% to as high as 30%.

Elevation (solar) The sun’s angle above the horizon.

Equalizing charge A controlled overcharge of a battery bank for the purpose of restoring equality of charge in all cells.

Finishing charge That part of the charging process which restores the final segment of a battery’s charge, roughly between 90% and 100% SOC.

Flat-plate array A photovoltaic (PV) array which does not use concentration.

Frequency The rate at which a periodic event occurs. In electricity, the rate at which current reverses direction in an alternating current system. In the US and some other countries, alternating current systems use a frequency of 60 cycles per second (60 Hz); in Europe and remaining countries, the standard is 50 Hz.

Global radiation Total solar radiant energy impinging on a surface, equal to the sum of direct and diffuse radiation.

Grid-connected A power system interconnected with the grid (or mains) of the local electric utility. Also referred to as utility-interactive.

Grid parity The point at which the cost of renewable electricity is equal to the cost of conventionally generated power/ electricity.

Heat Engine A device that converts thermal energy to mechanical output. The mechanical output is called work and the thermal energy input is called heat. Heat engines typically run on a specific thermodynamic cycle (Rankine, Stirling, etc.). Heat engines are often named after the thermodynamic cycle by which they are modeled. They often pick up alternate names, such as gasoline/petrol, turbine, or steam engines.

Hole An atom that has lost an electron.

Hybrid system A power system consisting of two or more energy sources (e.g., a photovoltaic (PV) array and a wind generator).

I-V curve A current/voltage curve, which expresses the possible combinations of current and voltage output of a photovoltaic device.

Insolation The solar energy received at a place over a given period. This may be expressed as peak sun-hours per day or kilowatt hours per square metre.

Inverter A device which converts DC electricity to AC.

Isolation diode A diode which prevents one segment of a photovoltaic (PV) array from interacting with another array segment. Usually used to prevent array energy from flowing backwards through a sub-voltage series string. May also serve the function of blocking diode.

Maximum power Also referred to as peak power. The point on a device’s I-V curve where the product of I and V (Pmax, measured in Watts) is maximized. The points on the I and V scales which describe this curve point are named Imp (current @ max power) and Vmp (voltage @ max power.)

Module A number of solar cells electrically connected, protected from environmental stresses, self-contained and not sub-dividable, providing a single electrical output.

Monocrystalline A silicon wafer with single crystal grain structure (isotropic) made using a CZ or float zone method.

Multicrystalline A silicon wafer which has multiple grain orientations. Often made using a modified directional solidification method.

n-type Silicon which has been doped, often with phosphorus, to have electrons as the majority electrical carriers.

NOCT Nominal Operating Cell Temperature; the temperature at which cells in a module operate under Standard Operating Conditions (SOC), which are: irradiance of 0.8 kW/m2, 20ºC ambient temperature, and average windspeed of 1 m/s, with the wind oriented parallel to the plane of the array, and all sides of the array fully exposed to the wind.

Open-circuit voltage Abbreviated Voc, refers to a photovoltaic device’s voltage potential when it is providing no current.

PPA Solar PPA is an agreement between a provider and a customer to purchase on-going solar power at long-term rates. Solar PPA providers install and maintain solar facilities on customer rooftops or properties. Customers pay only for the power generated by the facility—not solar equipment or installation—greatly reducing the risk and complications of implementing a solar electricity solution.

p-n junction The junction at the interface between two differently doped layers of semiconductor material, one layer doped with a positive-type dopant, the other layer with a negative-type dopant. An electrical field is established at the p-n junction which gives direction to the flow of light-stimulated electrons.

p-type Silicon which has been doped, often with boron or aluminium, to have positive charges as the majority electrical carriers.

Parallel connection Electrical connection where the positive terminals of a number of devices are connected together, as are their negative terminals. The output voltage of the paralleled devices is equal to the average of the devices, and the total current is the sum of the current of all the devices.

Passive Solar Technologies capture sunlight for heat energy without use of active mechanical systems such as fans or pumps (as contrasted to active solar). Such technologies convert sunlight into usable heat (water, air) or store heat for future use, with little or no use of other energy sources. A common example is a solarium on the sunny side of a building. Passive cooling is the use of the same design principles (no fans or pumps) to block solar heating to reduce summer cooling requirements.

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

Photovoltaic This is a term used to describe the solar electric effect.

Plasma enhanced chemical vapour deposition (PECVD) Method used to apply anti-reflection coating which enhances the electronic properties of the wafer.

Polycrystalline A silicon wafer which has multiple grain orientations. Often made using a modified directional solidification method.

R-Value A measure of thermal resistance used in the building and construction industry. A bigger number means the building insulation is more effective. The units of R-value are (hr * degree F*ft squared)/Btu. You can interpret R-Value units as the number of hours it takes for heat to flow across one square foot of assembly, where there is a 1 degree F temperature differential between the inside and outside temperatures. If an assembly of several components has a higher R-value it means that it poses more resistance to the flow of heat. A solar re-roof creates a warm solar air space above the old roof, which helps to increase the R value of the overall roof assembly.

Self-discharge Batteries lose charge even when not in use. For many batteries, self-discharge rate increases as the battery ages.

Series connection Electrical connection where the positive terminal of one device is attached to the negative terminal of the next in a series string; in this connection, the string voltage is the sum of the device voltages and the string current is limited to the current of the least productive device in the string.

Short-circuit current Abbreviated Isc, refers to a photovoltaic (PV) device’s current output when short-circuited.

Shunting path A short circuit path within a solar cell that will lead to efficiency loss.

Solar cell The smallest semiconductor device that can convert sunlight into electrical energy. Each silicon cell produces approximately half a volt, so many are wired in series to increase the voltage to significant levels.

Solar Energy The radiant energy from the Sun that influences Earth’s climate and weather and sustains life. It includes visible light and several non-visible frequencies such as ultraviolet light. All of these frequencies can be used to produce heat using solar thermal techniques and many can be used to produce electricity using photovoltaics.

Solar Energy Technologies Convert solar energy to useful purposes. They can provide: electrical generation by heat engine or photovoltaic means; space heating and cooling in active and passive solar buildings; potable water via distillation and disinfection; daylighting; hot water; thermal energy for cooking; and high temperature process heat for industrial purposes.

Solar module An assembly of solar cells in series or parallel to augment voltage and/or current.

Solar panel A group of solar modules mechanically mounted on a single frame or structure.

Solar spectrum The total distribution of electromagnetic radiant energy over the band of wavelengths present in solar radiation. The total energy received on a given surface, and how that energy is distributed among various wavelengths, depends on how much of the Earth’s atmosphere light has traversed.

Solar Thermal Power Systems Technologies using the sun’s heat to produce electricity or mechanical power. There are two kinds of systems. The most common is Concentrating Solar Thermal Power. A new system uses low temperature solar heat (without concentrating sunlight) to boil a special liquid that passes through a heat engine driving a generator to produce electrical power.

Standalone system A power system not connected to the utility grid (mains.) Sometimes referred to as an autonomous system.

Standard operating conditions Abbreviated SOC, a set of reference PV device measurement conditions consisting of irradiance of 0.8 kW/m2, 20ºC ambient temperature, and average windspeed of 1 m/s, with the wind oriented parallel to the plane of the array, and all sides of the array fully exposed to the wind.

Standard test conditions Abbreviated STC, a set of reference PV device measurement conditions consisting of irradiance of 1 kW/m2, AM 1.5, and 25ºC cell temperature.

Standoff mount A mounting system which supports a PV array above a roof surface.

State of charge Abbreviated SOC, the percentage of energy in a battery referenced to its nominal full capacity.

Sulfation The formation of lead sulfate crystals on the plates of a lead-acid battery. Normally used to refer to large sulfate crystals, rather than small crystals formed in normal battery operation, formed as a result of temperature cycling while the battery is in a partially charged state.

Texturing Part of the monocrystalline cell production process which helps to reduce the reflection of sunlight.

Thin-film cell A solar cell formed by depositing thin layers of conductive and semiconductive materials, usually using a chemical vapour deposition (CVD) process. Also referred to as amorphous cells because they have no crystalline structure. Such cells use less material than cells sawn from crystalline ingots.

Transpired Solar Collector A specialized solar collector that passes air through holes or openings in metal panels. Sunlight heats the metal panels. The outside air is heated as it moves through the openings in the panels. Other systems such as solar roofs of rustic shakes, shingles or tiles provide a similar effect but are not usually referred to as transpired collectors.

Two-axis tracking A tracking system which follows the sun’s azimuth and elevation.

U-Value Measurement of the heat flow that occurs during one hour through one square foot of material , when there is a 1 degree F temperature difference between the inside and outside air temperatures. The units are Btu/(hr *degreesF*ft squared ). If an assembly of several components has a higher U-value, it means that it loses more heat than one with a lower U-value.

Utility-interactive A power system which interacts with the utility grid (mains), taking power from the grid to satisfy its loads as necessary, and returning power to the grid when not required by the loads.

Voltage Measured in volts (V), the electrical potential between two points. One volt of potential causes one ampere of current to flow through a resistance of one ohm. The open-circuit voltage of a silicon solar cell is about half a volt; the operating voltage of a lead-acid cell is about two volts.

Wafer A slice of silicon brick used to make a solar cell.