Reference

Photovoltaic glossary

60 terms from photovoltaic engineering, installation practice, storage and electricity market design. Each entry is written to be useful on its own, with links to the article or calculator that treats the topic in full.

Showing 60 of 60 terms.

A

Air mass (AM)Measurement: The path length of sunlight through the atmosphere relative to the shortest possible path at sea level with the sun directly overhead. Standard test conditions use AM1.5, corresponding to a solar zenith angle of about 48 degrees, which approximates mid-latitude conditions.
AlbedoCells & modules: The fraction of incident light reflected by the ground surface beneath an array. Fresh snow reaches 0.8, white gravel roughly 0.4, grass and asphalt 0.1 to 0.2. Albedo drives the rear-side gain of bifacial modules and is a major variable in bifacial yield modelling. Read more
AmpacityElectrical: The maximum continuous current a conductor can carry without exceeding its insulation temperature rating, after applying derating factors for ambient temperature, grouping and installation method. Ampacity and voltage drop are separate checks; a cable must satisfy both. Read more
Anti-islandingGrid & policy: Inverter protection that disconnects the system when the grid supply is lost, so the inverter cannot energise a dead network and endanger utility workers. Required by IEC 62109-2, IEEE 1547 and EN 50549, and verified during commissioning.
ArrayCells & modules: The complete assembly of photovoltaic modules, strings, mounting structure and DC wiring that delivers power to one or more inverters. An array may be split into sub-arrays served by separate MPPT inputs.
AzimuthInstallation: The compass direction an array faces, usually expressed in degrees from true south in the northern hemisphere. Deviating 45 degrees east or west of optimum typically costs only 3 to 6 percent of annual yield in mid-latitudes, which is why east-west roofs remain viable. Read more

B

Balance of system (BOS)Installation: Everything in an installation that is not the modules: inverters, mounting, cabling, protection devices, monitoring, labour and design. BOS now dominates residential system cost in most European markets because module prices have fallen far faster than installation labour.
Bifaciality factorCells & modules: The ratio of rear-side to front-side efficiency of a bifacial module, typically 0.7 to 0.85 for n-type cells. Multiplied by rear irradiance, it gives the additional generation available from ground-reflected and diffuse light. Read more
Bypass diodeCells & modules: A diode wired across a substring of cells that conducts when those cells are shaded or damaged, providing an alternative current path. Without bypass diodes a single shaded cell would limit the whole string and overheat as a reverse-biased load, creating a hot spot. Read more

C

Capacity factorEconomics: Annual energy production divided by the energy the system would produce running at rated power for every hour of the year. Fixed-tilt PV in northern Europe reaches 10 to 12 percent; southern Spain and the US Southwest reach 18 to 22 percent.
Cell temperature coefficientCells & modules: The fractional change in a module parameter per degree of cell temperature above 25 degrees Celsius. Power coefficients run from about -0.45 percent per kelvin for older PERC designs to -0.24 percent per kelvin for heterojunction, which matters more than nameplate efficiency in hot climates. Read more
ClippingElectrical: The inverter limiting output when available DC power exceeds its AC rating. Modest clipping is deliberate: a DC/AC ratio of 1.15 to 1.35 raises annual yield per inverter euro because peak conditions occur for few hours a year. Read more
Combiner boxElectrical: An enclosure where multiple strings are paralleled onto a common DC bus, usually containing string fuses, surge protection and sometimes a DC isolator. Required once the number of parallel strings makes individual string protection necessary under IEC 62548.
CurtailmentGrid & policy: Deliberate reduction of export or generation, either by the operator in response to a grid signal or by a fixed export limit imposed as a connection condition. Curtailment losses must be modelled separately from technical losses in a financial case.

D

DC/AC ratioElectrical: Installed DC module capacity divided by inverter AC rating, also called the oversizing or inverter loading ratio. Values above one trade a few clipped peak hours for better inverter utilisation across the whole year. Read more
Depth of discharge (DoD)Storage: The fraction of a battery's usable capacity withdrawn in a cycle. Shallower cycling extends cycle life, which is why manufacturers specify warranty throughput at a stated DoD rather than a simple cycle count. Read more
DeratingInstallation: Reducing a component's permitted rating to account for real conditions: ambient temperature, cable grouping, installation method, altitude or soiling. Ignoring derating is one of the most common causes of overheating cables and nuisance tripping.
Diffuse irradianceMeasurement: Solar radiation scattered by the atmosphere and clouds rather than arriving directly from the solar disc. In northern Europe diffuse light supplies 50 to 60 percent of annual irradiation, which is why orientation penalties are milder there than intuition suggests.

E

Electroluminescence imaging (EL)Measurement: A diagnostic that forward-biases a module in darkness and photographs its infrared emission. Cracked cells, broken fingers and inactive areas appear dark. EL is the definitive test for transport damage and hail impact claims.

F

Feed-in tariff (FiT)Grid & policy: A fixed price paid per kilowatt-hour exported to the grid, usually guaranteed for a set contract period. Most European FiT schemes have been replaced by net billing or market-referenced export prices as PV became cost-competitive. Read more
Fill factor (FF)Cells & modules: The ratio of maximum power to the product of open-circuit voltage and short-circuit current. It measures how square the IV curve is; modern silicon cells reach 0.78 to 0.84. A falling fill factor in field measurements points to series resistance faults.

G

Global horizontal irradiance (GHI)Measurement: Total shortwave radiation received by a horizontal surface, the sum of direct and diffuse components. GHI is the baseline dataset from which plane-of-array irradiance is derived through transposition models.
Ground faultElectrical: An unintended current path between an active conductor and earth. In DC arrays a single ground fault may be undetected until a second one creates a fault loop, which is why residual current monitoring and insulation resistance testing are mandatory.

H

Heterojunction (HJT)Cells & modules: A cell architecture combining crystalline silicon with thin amorphous silicon passivation layers. It delivers excellent temperature coefficients, high bifaciality and low degradation, at the cost of a more expensive low-temperature production line. Read more
Hot spotCells & modules: A localised overheating region where a reverse-biased cell dissipates power produced by the rest of the string. Caused by shading, cracks or mismatch. IEC 61215 includes a hot-spot endurance test; thermography finds them in the field.

I

Insulation resistance testMeasurement: A commissioning measurement applying DC test voltage between array conductors and earth to confirm insulation integrity. IEC 62446-1 specifies test voltages and minimum acceptable values, which depend on system voltage. Read more
InverterElectrical: The device converting array DC into grid-compliant AC, while performing maximum power point tracking, grid protection and monitoring. It is the component most likely to need replacement during a system's life. Read more
IV curveMeasurement: The current-voltage characteristic of a cell, module or string under given irradiance and temperature. Its knee is the maximum power point. Field IV tracing distinguishes shading, mismatch, series resistance and shunt faults by curve shape. Read more

L

LeTIDCells & modules: Light and elevated temperature induced degradation, a slower loss mechanism appearing over months in some PERC modules operating hot. Unlike LID it can partially recover, and manufacturers now apply stabilisation treatments to suppress it.
Levelised cost of electricity (LCOE)Economics: Lifetime discounted cost divided by lifetime discounted energy production, giving a cost per kilowatt-hour comparable across technologies. LCOE is highly sensitive to the discount rate and to assumed degradation, so quoted figures require their assumptions. Read more
LFPStorage: Lithium iron phosphate battery chemistry. Lower energy density than nickel-based chemistries but longer cycle life, better thermal stability and no cobalt, which has made it the default for stationary home storage. Read more
Light-induced degradation (LID)Cells & modules: An initial power loss of roughly 0.5 to 3 percent occurring in the first hours of sun exposure in boron-doped p-type silicon, caused by boron-oxygen complexes. Gallium doping and n-type wafers largely eliminate it. Read more

M

Maximum power point tracking (MPPT)Electrical: The control algorithm that continuously adjusts operating voltage to keep a string at the knee of its IV curve as irradiance and temperature change. Independent MPPT inputs let differently oriented or shaded sub-arrays operate at their own optimum. Read more
MicroinverterElectrical: A small inverter mounted at each module, converting to AC at the panel. Eliminates high-voltage DC on the roof and gives per-module tracking and monitoring, at higher cost per watt and with more units exposed to weather. Read more
Mismatch lossElectrical: Energy lost because series-connected modules do not share identical characteristics, so the string operates below the sum of individual maxima. Sorting by current bin, avoiding mixed module types in a string and limiting shading all reduce it.
Module-level power electronics (MLPE)Electrical: Collective term for microinverters and DC optimisers, which perform tracking or conversion at the module rather than the string. Valuable on complex or shaded roofs and where rapid shutdown is mandated.

N

Net billingGrid & policy: An export scheme that values exported energy at a different, usually lower, rate than imported energy. It replaces the one-for-one kilowatt-hour offset of net metering and makes self-consumption and storage economically significant. Read more
Net meteringGrid & policy: A billing arrangement where exported kilowatt-hours offset imported ones at the same value over a settlement period. Generous for the owner, increasingly withdrawn as PV penetration grows and cost-shifting concerns rise.
NMCStorage: Nickel manganese cobalt lithium-ion chemistry. Higher energy density and better low-temperature behaviour than LFP, with shorter cycle life, higher cost and stricter thermal management requirements.
NOCTMeasurement: Nominal operating cell temperature, measured at 800 W/m² irradiance, 20 degrees Celsius ambient and 1 m/s wind. Typically 41 to 45 degrees. It is a better basis for field expectations than the 25 degree standard test condition.

O

Open-circuit voltage (Voc)Electrical: String voltage with no current flowing. It rises as temperature falls, so the design check that fixes maximum modules per string is Voc at the coldest expected cell temperature against the inverter's maximum input voltage. Read more
OptimiserElectrical: A DC-DC converter at each module that performs module-level tracking and feeds a conventional string inverter. Retains a central inverter for conversion while recovering most shading and mismatch losses.

P

Payback periodEconomics: The time for cumulative net savings to equal the installed cost. Simple payback ignores the time value of money, escalating tariffs and degradation; a discounted calculation is more honest and usually one to three years longer. Read more
PERCCells & modules: Passivated emitter and rear cell, the dielectric rear-passivation architecture that dominated the market through the late 2010s. Now being displaced by TOPCon and heterojunction as the mainstream high-volume technology. Read more
Performance ratio (PR)Measurement: Measured energy divided by the energy expected from plane-of-array irradiance at nameplate efficiency. Well-designed European rooftop systems achieve 0.78 to 0.85. A falling PR over time is the primary indicator of a developing fault.
Plane-of-array irradiance (POA)Measurement: Irradiance in the tilted plane of the modules, derived from horizontal data through a transposition model plus ground reflection. All performance ratio calculations should use POA, not GHI.
Potential-induced degradation (PID)Cells & modules: Power loss caused by leakage currents driven by high system voltage between cells and the grounded frame, aggravated by heat and humidity. Mitigated by PID-resistant encapsulants, grounding schemes and anti-PID inverter functions.

R

Rapid shutdownInstallation: A requirement to reduce conductor voltage inside and around an array to a safe level within seconds of activation, for firefighter safety. Mandated in the United States by NEC 690.12 and implemented in Europe through various national and insurer requirements.

S

Self-consumption ratioEconomics: The share of generated energy used on site rather than exported. Under net billing it is the main lever on financial return, because avoided import is worth substantially more than export revenue. Read more
Short-circuit current (Isc)Electrical: Current with the terminals shorted, proportional to irradiance. Protection and conductor sizing use 1.25 times Isc to allow for irradiance enhancement above 1000 W/m² from cloud edge effects.
Soiling lossInstallation: Energy lost to dust, pollen, salt, bird droppings and industrial deposits on module glass. Typically 1 to 3 percent annually in temperate rainy climates and 5 to 15 percent in arid dusty regions where rainfall does not clean the array. Read more
Standard test conditions (STC)Measurement: 1000 W/m² irradiance, 25 degrees Celsius cell temperature, AM1.5 spectrum. The laboratory basis for nameplate ratings, and conditions that a roof array essentially never experiences, since 1000 W/m² usually comes with hot cells.
StringElectrical: A group of modules connected in series so their voltages add at a common current. String length is bounded above by cold-weather Voc and below by the inverter's minimum MPPT voltage at high temperature.
Surge protective device (SPD)Installation: A component that diverts transient overvoltage to earth, protecting inverters and monitoring equipment from lightning-induced surges. Type and placement follow the risk assessment in IEC 62305 and the array's cable route lengths. Read more

T

ThermographyMeasurement: Infrared inspection of an operating array to find hot spots, failed bypass diodes, disconnected substrings and poor terminations. Requires at least 600 W/m² irradiance and a known view angle to be interpretable. Read more
Tilt angleInstallation: The angle between the module plane and the horizontal. Annual-yield optimum is roughly latitude minus 10 to 15 degrees in mid-latitudes, but self-consumption, snow shedding and wind loading often justify departing from it.
TOPConCells & modules: Tunnel oxide passivated contact, an n-type architecture using a thin oxide plus doped polysilicon layer to reduce recombination at the rear contact. It has become the mainstream successor to PERC because it upgrades existing production lines.
Transposition modelMeasurement: An algorithm converting horizontal irradiance into plane-of-array irradiance, accounting for the anisotropy of diffuse sky radiation. Perez and Hay-Davies are the common implementations, and model choice moves annual yield estimates by a few percent.

V

Voltage dropElectrical: The voltage lost in conductor resistance between source and load, expressed in volts or as a percentage of nominal voltage. Design practice keeps DC array drop below 1 percent and the AC run below about 3 percent. Read more

Y

YieldMeasurement: Annual energy production per installed kilowatt, in kWh/kWp. Roughly 850 to 1,000 in northern Europe, 1,100 to 1,350 in central and southern Europe, and above 1,600 in the sunniest parts of the Mediterranean and the US Southwest.