NABCEP SW Associate key terms

The instantaneous power density of sunlight falling on a surface, measured in watts per square meter. It determines how much current a module can produce at a given moment.

The accumulated solar energy delivered per unit area over a period of time, expressed in kilowatt-hours per square meter. It is the energy total that irradiance adds up to.

A laboratory reference set of 1000 W/m² irradiance, 25 °C cell temperature, and a specified air mass spectrum used to rate module power. Real installations rarely operate at these values.

The daily solar energy received divided by 1000 W/m², expressed as equivalent hours at full irradiance. It is a convenient shortcut for estimating daily array production.

A measure of how much atmosphere sunlight passes through before reaching a surface, with 1.0 being the sun directly overhead. Higher values shift the spectrum and reduce available energy.

The compass direction of the sun measured in degrees clockwise from true north. It changes through the day and season and guides array orientation choices.

The sun's angular height above the horizon, ranging from 0° at sunrise and sunset to a maximum at solar noon. It controls how much sunlight a tilted surface receives.

The angle between a module surface and horizontal. It affects seasonal energy capture, snow shedding, and self-cleaning by rain.

The generation of voltage and current when light strikes a semiconductor junction. Absorbed photons free charge carriers that are separated by the built-in electric field.

The boundary between p-type and n-type semiconductor layers within a solar cell. The resulting internal electric field separates light-generated electrons and holes to produce usable current.

The maximum voltage a module produces with no load connected, abbreviated Voc. It rises as cell temperature falls, so cold-weather design must account for it.

The current that flows when module terminals are connected through a near-zero resistance, abbreviated Isc. It scales roughly with irradiance and is used for conductor and fuse sizing.

The operating voltage and current pair where a module delivers its highest power. Inverters continuously track this point as irradiance and temperature change.

The ratio of a module's maximum power to the product of its open-circuit voltage and short-circuit current. A higher value indicates lower internal losses and a better-quality cell.

The percentage change in module power for each degree Celsius away from 25 °C. It is negative for power, so hot modules produce less than their nameplate rating.

The power output a manufacturer guarantees under standard test conditions, typically printed on the module label. It is a reference value, not a prediction of field output.

Electric current that reverses direction periodically, typically 60 Hz in North America. Grid-tied inverters convert array direct current into this form for loads and the utility.

Electric current that flows in one direction only. Solar modules and batteries produce direct current, which inverters or charge controllers then manage.

The relationship stating that voltage equals current multiplied by resistance. It underlies voltage drop calculations, conductor sizing, and troubleshooting measurements.

The maximum current a conductor can carry continuously without exceeding its temperature rating. It depends on conductor size, insulation, ambient temperature, and installation method.

The reduction in voltage along a circuit caused by conductor resistance. Excessive drop wastes energy and can cause equipment to operate below its rated voltage.

An inverter or charge controller function that continuously adjusts operating voltage to keep the array at its peak power point as conditions vary.

A inverter fed by one or more series-connected module strings, with all modules on a string operating at a shared current. Shading or mismatch on one module affects the whole string.

A small inverter mounted at or near a single module that converts its direct current to alternating current locally. It reduces mismatch losses and simplifies module-level monitoring.

A diode wired across groups of cells inside a module that provides an alternate current path when cells are shaded or faulty. It limits hot-spot heating and power loss.

A diode placed in series with an array or battery circuit to prevent reverse current flow at night or from other parallel strings. It is common in battery-based systems.

A localized overheated area in a module caused by a cell that is shaded, damaged, or mismatched while the rest of the string drives current through it. Severe cases can crack glass or start fires.

The process of evaluating obstructions and their seasonal shadow paths to predict production losses and choose array locations. Tools include site walks, sun path charts, and shade measurement devices.

The ratio of actual energy produced to the energy a system would produce at its rated power under the same irradiance. It captures losses from temperature, soiling, wiring, and conversion.

A device that regulates current from modules to a battery bank to prevent overcharging and, in some designs, over-discharging. Maximum power point tracking types harvest more energy.

The percentage of a battery's capacity that has been withdrawn from a fully charged state. Limiting it extends battery life and affects usable storage sizing.

The remaining energy in a battery expressed as a percentage of its full capacity. It is the complement of depth of discharge at any given moment.

A fuse or circuit breaker that interrupts current above a safe level to protect conductors and equipment. PV source circuits require devices rated for direct current and appropriate voltage.

A system that detects current leaking from an energized conductor to ground and interrupts the circuit. It reduces shock and fire hazards in photovoltaic arrays.

A conductor that bonds non-current-carrying metal parts of a system together and to ground. It provides a low-impedance path for fault current.

A required function that reduces voltage at module-level conductors within a defined time after activation, protecting firefighters. It typically uses module-level electronics or a shunt trip.

Devices such as microinverters or direct-current optimizers mounted at each module. They enable per-module tracking, monitoring, and rapid shutdown compliance.

An unintended electrical discharge across a gap in a damaged or loose connection. Direct-current arc faults in photovoltaic systems can persist and ignite surrounding materials.

A term for the solar energy received on a surface over time, often used interchangeably with irradiation in industry practice. Values guide production estimates and array sizing.