Solar dictionary

Learn everything to know about solar energy including the terminology

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Alternating current. AC is the type of electricity that powers your home. Solar panels produce DC (Direct Current) which must be converted to AC before it can be used to power your home.


A unit of the flow of electrical current. One volt of power across one ohm of resistance equals a current flow of one amp


The measure of the flow of current (in Amps) over one hour. It is used to measure battery capacity

Annual solar savings

The annual potential savings from solar is the amount of energy that could potentially be saved by going solar vs the energy requirements of the building


Photovoltaic array, an array made up of photovoltaic solar cells

Array current

The amount of current (in Amps) produced by a solar array when exposed to sunlight

Array operating voltage

The voltage produced by a solar array when exposed to sunlight



The average amount of power that a utility company must supply during a given period of time.

Baseload generating plants

Currently this refers to coal, gas, or nuclear power plants that operate at nearly constant levels. They operate to maintain the amount of power needed from the grid.


Storage cells that store power from either solar or wind power and then they provide that power as needed.

Battery available capacity

The maximum amount of power expressed in Amp-hours that a battery can provide. The battery’s available capacity will be affected by its discharge rate, temperature, initial state of charge. Age, and cut off voltage.

Battery capacity

The maximum amount of power a battery can supply to a load under a specific set of conditions. These conditions can include temperature, battery age, and cut off voltage.

Battery life

The number of charge/discharge cycles a battery can provide before it’s storage capacity drops below a certain level.

Blocking diode

An electrical component that allows power to flow from solar panels to batteries, but blocks power from flowing from the batteries to the solar cells. The blocking diode prevents the batteries from discharging themselves into the solar panels.

British Thermal Unit (BTU)

The amount of heat required to raise the temperature of one pound of water one degree Fahrenheit.

Bypass diode

A diode connected across one or more solar cells in a solar panel that will protect that solar cell from thermal destruction in the event that the cell is shaded while others are exposed to sunlight.


Cathodic protection

A method for preventing corrosion caused by oxidation of exposed metal by imposing a small electrical voltage between that metal and ground.

Charge controller

A component in a solar system with batteries that controls the flow of current to and from the batteries. The charge controller protects the batteries from over-charging and over-discharging.

Charge factor

The time measured in hours that a battery can be charged at a constant level of current without damage to the battery. It is usually expressed in relation to the battery’s capacity. For example, C/5 indicated a charge factor of 5 hours.

Charge rate

The rate that a battery should be charged to bring it up to its full capacity

Cloud enhancement

The increase in solar intensity caused by reflected irradiance from nearby clouds


The material that electricity travels through, usually wire or a trace on a circuit board.


A component that converts DC voltage to another DC voltage


Electric current, measured in Amps

Cutoff voltage

The voltage level that the charge controller disconnects the solar array from the battery or disconnects the load from the battery.


The discharge and recharge of a battery


DC to DC converter

A component that converts DC voltage to another DC voltage

Deep cycle battery

A battery designed to withstand many discharges to 20% or less of its charge capacity

Deep discharge

Discharging a battery to 20% or less of its full charge

Demand response

A voluntary reduction in electrical usage during peak demand hours

Depth of discharge

A measure of the Amp-hours drained from a battery expressed as a percentage of its rated capacity. For example, a discharge of 25 Amp-hours from a 100 Amp-hour battery results in a depth of discharge of 25%

Design month

The month that has the combination of insolation and load that requires maximum output from a solar system.

Direct current (DC) 

Electric current that flows in a single direction, unlike AC current which flows in both directions.

Direct insolation

Sunlight that falls directly on a solar panel


The act of withdrawing electricity from a battery

Discharge rate

The rate in Amp-hours that electricity is withdrawn from a battery

Distributed generation

A method of producing power close to where it is used which helps reduce the need for electrical transmission lines

Distributed power

A term for any power supply such as a battery farm that is situated close to where power is used.

Distributed systems 

Power producing systems such as residential solar systems, that are situated at or near where the power they generate is used.


Time when a solar system can’t provide power. It is usually expressed in terms of hours per year or as a percentage

Duty cycle

The ratio of time vs a 24 hour day that an appliance or another electrical load is drawing power.

Duty rating

The amount of time that an inverter can produce it’s full rated power level.


Electric circuit

A path followed by electricity composed of either wires or metal traces on a circuit board.

Electric current

The flow of electricity in a circuit measured in Amps

Electric grid

Also known as “the grid” is the system of power sources and transmission wires that supply electrical power to homes and businesses.


A force that can be harnessed to do useful work. In solar energy, that force is electricity derived from the energy of the sun.

Energy audit

A survey usually conducted by a utility company that shows how much energy is used in a home. It also includes ways to reduce energy usage.

Energy density

The ratio of energy available from a battery per pound of battery


Flat-plate photovoltaics

A solar system that consists of flat solar cells that absorb direct and indirect sunlight. This is the type of solar array installed on most homes

Float charge

The voltage required to maintain the charge of a battery against self-discharge at any given temperature

Float life

The number of years that a battery can keep its stated capacity when it is kept at float charge.

Float service

The process of supplying a battery with a float charge sufficient to keep it fully charged.


The rate at which a waveform will oscillate between its highest peak and lowest trough. It is also expressed as cycles per second. For example, 120 volts AC current has a frequency of 60 cycles per second.

Full sun

The amount of power received by a solar system when the sun is closest to directly overhead at noon during the day.



1 billion watts of power.


The system of power sources and transmission wires that supply electrical power to homes and businesses.

Grid-connected system

A solar system that is connected to the electrical grid and supplies electricity to it.

Ground mount system

A solar system that is mounted in a frame that is connected to the ground, rather than the roof.


High voltage disconnect

The voltage at which a solar system’s charge controller will disconnect the solar system from its batteries to prevent overcharging.

Hybrid system

A solar system that includes other sources of power generation such as wind generators


Incident light

Light that shines onto the face of a solar panel

Independent System Operator (ISO)

The entity responsible for maintaining system balance, grid reliability, and electricity market operation

Infrared radiation

Energy in the form of electromagnetic radiation that is invisible to the naked eye. It is often felt as heat. Solar panels can generate power from infrared radiation, but it’s a lot less effective than visible light.


A measurement of the amount of energy that falls on a solar panel from the sun. Insolation is measured in watts per square meter or BTU’s per square foot per hour.


A device that converts DC current into AC current.


The direct, diffuse, and reflected solar radiation that strikes a surface. Usually expressed in kilowatts per square meter. Irradiance multiplied by time equals insolation.


Junction box

A solar system junction box is an enclosure on the module where solar panels are electrically connected and where protection devices can be located, if necessary.



1,000 watts


1,000 thousand watts acting over a period of one hour.


Levelized cost of energy

The cost of energy of a solar system that is based on the system’s installed price, its total lifetime cost, and its lifetime electricity production.


The period of time that a solar system can produce power above a certain specified minimum performance level

Life cycle cost

The estimated cost of owning and operating a solar system for the length of its useful life.

Line commutated inverter

An inverter that is tied to a power grid or line. An inverter that is controlled by the power line, so that, if there is a failure in the power grid, the solar system cannot feed power into the line. This type of inverter is required by law to be used in grid-connected solar systems


The demand on an energy-producing system; the energy consumption or requirement of a piece or group of equipment. Usually expressed in terms of amps or watts.

Load circuit

The wire, switches, fuses, and other components that connect the load to the power source.

Load current

The amount of current that a device requires to operate.

Load resistance

The electrical resistance presented by the load.

Low voltage cutoff

The voltage level at which a charge controller will disconnect the load from the battery.

Low voltage disconnect

Same as above

Low voltage disconnect hysteresis

The voltage difference between the low voltage cutoff point and the level of voltage at which the load will be reconnected

Low voltage warning

A warning buzzer or light that indicates the low battery voltage disconnect point has been reached.


Maximum power point

The point on the current-voltage (I-V) curve of a module under illumination, where the product of current and voltage (Amps times Volts) is maximum.

Maximum Power Point Tracker

A method where a power conditioning unit automatically operates a solar system at its maximum power point under all conditions.

Maximum power tracking

A means of operating a solar system at the peak power point of the arrays Amps vs Voltage curve such that maximum power output is obtained. Also known as peak power tracking.


One million watts or 1,000 kilowatts.

Megawatt hour

One million watt-hours or 1,000 kilowatt-hours.

Multi-stage controller

A battery charge controller that allows different charging currents as the battery nears its full state of charge.


National Electrical Code (NEC)

Contains guidelines for all types of electrical installations. Article 690 of the NEC contains, “Solar Photovoltaic Systems” which should be followed when installing a PV system.

National Electrical Manufacturers Association (NEMA)

Organization that sets standards for some non-electric parts such as junction boxes.


National Electrical Code


National Electrical Manufacturers Association

Nominal voltage

A reference voltage used to give the capacity in volts for batteriessolar panels or other electricity supplying devices, e.g 12 volts or 24-volt battery or system.



A measure of electrical resistance

Open circuit voltage

The maximum possible voltage available from a solar cell when no current is flowing.

Operating point

The current and voltage produced by a battery or solar system when connected to a load.


Placement with respect to the cardinal directions, N, S, E, W; azimuth is the measure of orientation from north.


Attempting to force current into a fully charged battery. Overcharging can damage batteries.


Packing factor

The ratio of array area to actual land area or building envelope area for a system; or, the ratio of total solar cell area to the total module area, for a module.


Solar panel

Parallel connection

A way of joining batteries or solar panels together by connecting positive leads together and negative leads together. A parallel connection increases amperage but not voltage.

Peak demand/load

The maximum energy demand or load in a given time.

Peak power current

Operating point of a solar cell where the product of the current times the voltage is at its maximum value (I * V)

Peak power point

Same as above

Peak power tracking

A means of operating a solar system at the peak power point of the array’s Amps vs Voltage curve such that maximum power output is obtained. Also known as peak power tracking.

Peak sun hours

The equivalent number of hours per day when solar irradiance averages 1,000 w/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 1,000 w/m2.

Peak watt

Used as a measure of the performance of solar panels or solar systems. The maximum nominal output of the device in watts under standardized test conditions.


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

Photovoltaic(s) (PV)

Components that perform the direct conversion of light into electricity.

Photovoltaic (PV) array

An interconnected system of PV modules that function as a single electricity-producing unit. The modules are assembled as a discrete structure, with common support or mounting.

Photovoltaic (PV) cell

The smallest semiconductor element within a PV module to perform the immediate conversion of light into electrical energy (direct current voltage and current). Also called a solar cell.

Photovoltaic (PV) 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) device

A solid-state electrical device that converts light directly into direct current electricity.

Photovoltaic (PV) effect

The phenomenon that occurs when photons, the “particles” in a beam of light, knock electrons loose from the atoms they strike. When this property of light is combined with the properties of semiconductors, electrons flow in one direction across a junction, setting up a voltage. With the addition of circuitry, a current will flow and electric power will be available.

Photovoltaic (PV) generator

The total of all PV strings of a PV power supply system, which are electrically interconnected.

Photovoltaic (PV) module

The smallest environmentally protected, essentially planar assembly of solar cells and ancillary parts, such as interconnections, terminals, (and protective devices such as diodes) intended to generate direct current power under unconcentrated sunlight.

Photovoltaic (PV) panel

often used interchangeably with PV module (especially in one-module systems), but more accurately used to refer to a physically connected collection of modules.

Photovoltaic (PV) system

A complete set of components for converting sunlight into electricity by the photovoltaic process, including the array and balance of system components.

Plug-and-play PV system

A commercial, off-the-shelf photovoltaic system that is fully inclusive with little need for individual customization. The system can be installed without special training and using few tools. The homeowner plugs the system into a PV-ready circuit and an automatic PV discovery process initiates communication between the system and the utility. The system and grid are automatically configured for optimal operation.


The amount of electrical energy available for doing work, measured in horsepower, Watts, or Btu per hour.

Power conditioning

The process of modifying the characteristics of electrical power (for e.g., inverting direct current to alternating current).

Power conditioning equipment

Electrical equipment, or power electronics, used to convert power from a photovoltaic array into a form suitable for subsequent use. A collective term for inverterconverter, a battery charge regulator, and blocking diode.

Power conversion efficiency

The ratio of output power to the input power of the inverter.

Power density

The ratio of the power available from a battery to its mass (W/kg) or volume (W/l).

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-amps or kilovolt-amps.

Pulse-width-modulated (PWM) wave inverter

A type of power inverter that produces a high-quality voltage, at minimum current harmonics (current distortions).



Rated battery capacity

The term used by battery manufacturers to indicate the maximum amount of energy that can be withdrawn from a battery under specified discharge rate and temperature. See also battery capacity.

Rated module current (A)

The current output of a photovoltaic module measured at standard test conditions of 1,000 w/m2 and 25°C cell temperature.

Rated power

Rated power of the inverter. However, some units can not produce rated power continuously. See also duty rating.


A device that converts alternating current to direct current. See also inverter.


Prevents overcharging of batteries by controlling charge cycle-usually adjustable to conform to specific battery needs.

Resistance (R)

The property of a conductor, which opposes the flow of an electric current resulting in the generation of heat in the conducting material. The measure of the resistance of a given conductor is the electromotive force needed for a unit of current flow. The unit of resistance is ohms.

Reverse current protection

Any method of preventing unwanted current flow from the battery to the photovoltaic array (usually at night). See also blocking diode.


See root mean square.

Root mean square (RMS)

The square root of the average square of the instantaneous values of an ac output. For AC power, the RMS value is 0.707 times the peak value. The equivalent value of alternating current, I, that will produce the same heating in a conductor with resistance, R, as a dc current of value I.


Sacrificial anode

A piece of metal buried near a structure that is to be protected from corrosion. The metal of the sacrificial anode is intended to corrode and reduce the corrosion of the protected structure.


The rate at which a battery, without a load, will lose its charge.

Series controller

charge controller that interrupts the charging current by open-circuiting the photovoltaic (PV) array. The control element is in series with the PV array and battery.

Series regulator

Type of battery charge regulator where the charging current is controlled by a switch connected in series with the photovoltaic module or array.

Shelf life of batteries

The length of time, under specified conditions, that a battery can be stored so that it keeps its guaranteed capacity.

Short-circuit current

The current flowing freely through an external circuit that has no load or resistance; the maximum current possible.

Shunt controller

charge controller that redirects or shunts the charging current away from the battery. The controller requires a large heat sink to dissipate the current from the short-circuited photovoltaic array. Most shunt controllers are for smaller systems producing 30 amperes or less.

Shunt regulator

Type of a battery charge regulator where the charging current is controlled by a switch connected in parallel with the photovoltaic (PV) generator. Shorting the PV generator prevents overcharging of the battery.

Sine wave inverter

An inverter that produces utility-quality, sine wave power forms.

Single-stage controller

charge controller that redirects all charging current as the battery nears full state-of-charge.

Smart grid

An intelligent electric power system that regulates the two-way flow of electricity and information between power plants and consumers to control grid activity.

Soft costs

non-hardware costs related to PV systems, such as financing, permitting, installation, interconnection, and inspection.

Solar cooling

The use of solar thermal energy or solar electricity to power a cooling appliance. Photovoltaic systems can power evaporative coolers (“swamp” coolers), heat-pumps, and air conditioners.

Solar energy

Electromagnetic energy transmitted from the sun (solar radiation). The amount that reaches the earth is equal to one billionth of total solar energy generated, or the equivalent of about 420 trillion kilowatt-hours.

Solar insolation

See insolation.

Solar irradiance

See irradiance.

Solar noon

The time of the day, at a specific location, when the sun reaches its highest, apparent point in the sky.

Solar panel

See photovoltaic (PV) panel.

Solar resource

The amount of solar insolation a site receives, usually measured in kWh/m2/day, which is equivalent to the number of peak sun hours.

Standby current

This is the amount of current (power) used by the inverter when no load is active (lost power). The efficiency of the inverter is lowest when the load demand is low.

Stand-off mounting

Technique for mounting a photovoltaic array on a sloped roof, which involves mounting the modules a short distance above the pitched roof and tilting them to the optimum angle.

State-of-charge (SOC)

The available capacity remaining in the battery, expressed as a percentage of the rated capacity.

Storage battery

A device capable of transforming energy from electric to chemical form and vice versa. The reactions are almost completely reversible. During discharge, chemical energy is converted to electric energy and is consumed in an external circuit or apparatus.


A number of photovoltaic modules or panels interconnected electrically in series to produce the operating voltage required by the load.


Any one of several components in a photovoltaic system (i.e., array, controller, batteries, inverter, load).


Tare loss

Loss caused by a charge controller. One minus tare loss, expressed as a percentage, is equal to the controller efficiency.

Temperature compensation

A circuit that adjusts the charge controller activation points depending on battery temperature. This feature is recommended if the battery temperature is expected to vary more than ±5°C from the ambient temperature.

Temperature factors

It is common for three elements in photovoltaic system sizing to have distinct temperature corrections (changes due to temperature): a factor used to decrease battery capacity at cold temperatures; a factor used to decrease PV module voltage at high temperatures, and a factor used to decrease the current carrying capability of wire at high temperatures.

Tilt angle

The angle at which a photovoltaic array is set to face the sun relative to a horizontal position.

Total AC load demand

The sum of the alternating current loads. This value is important when selecting an inverter.


An electromagnetic device that changes the voltage of alternating current electricity.

Trickle charge

A charge at a low rate, balancing through self-discharge losses, to maintain a battery in fully-charged condition.


Uninterruptible power supply (UPS)

The designation of a power supply providing continuous uninterruptible service. The UPS will contain batteries.

Utility-interactive inverter

An inverter that can function only when tied to the utility grid, and uses the prevailing line-voltage frequency on the utility line as a control parameter to ensure that the photovoltaic system’s output is fully synchronized with the utility power.


Volt (V)

A unit of electrical force equal to that amount of electromotive force that will cause a steady current of one ampere to flow through a resistance of one ohm.


The amount of electromotive force, measured in volts, that exists between two points.

Voltage at maximum power (Vmp)

The voltage at which maximum power is available from a photovoltaic module.

Voltage protection

Many inverters have sensing circuits that will disconnect the unit from the battery if input voltage limits are exceeded.

Voltage regulation

This indicates the variability in the output voltage. Some loads will not tolerate voltage variations greater than a few percent.



The rate of energy transfer equivalent to one ampere under an electrical pressure of one volt. One watt equals 1/746 horsepower. It is the product of voltage and current (amperage).


Watts per meter squared




Zenith angle

the angle between the direction of interest (of the sun, for example) and the zenith (directly overhead).

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