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Glossary

55 solar and electrical terms, from STC to temperature coefficient, defined without the jargon.

Method shownReviewed August 2026Our methodology

55 terms

Panels

Bifaciality factor: For a bifacial module, a declared ratio comparing rear-side and front-side response under the stated test basis. It is not a site-yield forecast: rear irradiance, mounting, albedo, shading and mismatch determine the installed rear-side contribution.
Busbars: The thin conductive lines on a cell's surface that collect the current the fine gridlines gather and carry it to the cell interconnects. More, thinner busbars (multi-busbar / MBB designs) shorten the path current travels, reducing resistive losses and shading.
HJT (Heterojunction): Heterojunction Technology: a cell architecture combining crystalline silicon with thin amorphous-silicon layers for surface passivation. Compare finished modules using their exact datasheets and warranties rather than carrying generic HJT performance figures into a design.
Light-Induced Degradation (LID): A change in crystalline-silicon module performance associated with initial light exposure. The mechanism and magnitude depend on the cell materials and processing, so use product-specific qualification and warranty evidence rather than assigning a technology-wide loss.
Module efficiency: The share of incident solar power a whole module converts to electrical power under its stated rating conditions. Higher efficiency means more rated watts can fit into the same module area; compare exact models on the same test basis.
NOCT / NMOT (Nominal Operating Cell Temperature): A manufacturer-declared operating-temperature result measured under stated irradiance, ambient temperature, wind and mounting conditions. It is useful for comparison, but the printed test method and conditions must be read from the exact datasheet rather than treated as a site forecast.
PERC: Passivated Emitter and Rear Cell: a crystalline-silicon cell architecture with rear-surface passivation intended to reduce recombination and improve light capture. The acronym does not establish a finished module's efficiency, degradation or warranty.
Potential-Induced Degradation (PID): A module-degradation mechanism associated with electrical potential between the cell circuit and other module or system parts. Susceptibility depends on module construction, system configuration and environmental conditions; use exact qualification and diagnostic evidence rather than assuming PID from low output alone.
Power tolerance: How far an individual panel's actual power may vary from its nameplate rating. A positive-only tolerance (e.g. 0/+5 W) guarantees you never get less than the label; a ± tolerance means some panels ship under-rated.
Standard Test Conditions (STC): The reference conditions used for PV module nameplate ratings: 1000 W/m² irradiance, 25 °C cell temperature and an AM1.5 solar spectrum. STC supports comparison on a common basis; it is not a forecast of output on a particular roof.
Temperature coefficient of Pmax: How much a panel's maximum power changes per degree Celsius of cell temperature relative to the datasheet reference, expressed in %/°C. Power normally falls as cells heat up. Use the exact model's published coefficient rather than a generic value for its cell technology.
Temperature coefficient of Voc: How much a panel's open-circuit voltage changes per degree Celsius relative to 25 °C, in %/°C. Voltage rises as cells get colder, so on a frosty morning Voc can climb above its rated value. Installers use this to ensure a cold-weather string never exceeds the inverter's maximum input voltage.
TOPCon: Tunnel Oxide Passivated Contact: a silicon-cell contact architecture using an ultra-thin oxide and doped silicon layer to reduce recombination at the contact. It does not by itself establish a module's efficiency, temperature behaviour, degradation or warranty.

Inverters

CEC / weighted inverter efficiency: An inverter-efficiency metric calculated from performance at several load points using a defined weighting, rather than the single best conversion point. Compare values only when they use the same published method and test boundary.
DC-to-AC ratio (inverter loading ratio): The ratio of an array's rated DC power to the inverter's rated AC output. A higher ratio can increase inverter utilisation and clipping; the suitable value depends on the exact equipment limits, site conditions and design requirements.
Hybrid inverter: An inverter that combines solar-inverter and battery-inverter or charger functions. Backup capability is product- and design-specific, so the hybrid label alone does not establish what loads can operate during a grid outage.
Inverter clipping: When available array power is above the inverter's conversion or output limit, the inverter caps output and the excess is not delivered. The expected frequency and energy impact depend on the DC-to-AC ratio, site conditions and exact inverter behaviour.
Maximum Power Point Tracking (MPPT): A control technique in inverters and charge controllers that continuously adjusts the array's operating voltage and current to keep it at the point that yields the most power as irradiance, temperature, and shading change. Multiple independent MPPT inputs let different roof faces or string lengths be optimised separately.
Microinverter: An inverter installed at an individual PV module that converts that module's DC output to AC. This provides module-level conversion; monitoring, shutdown, backup and fault behaviour depend on the complete product and system design.
Power optimiser: A module-level DC-DC power-electronics device used to condition a panel's output before a central inverter. Its operating, monitoring and compatibility requirements depend on the exact optimiser, module and inverter combination.
String inverter: An inverter that converts the combined DC output of one or more series-connected PV strings into AC. Panels on a shared input operate within the inverter's MPPT arrangement, so string layout, mismatch and shading must be assessed for the exact design.

Batteries

C-rate: A measure of how fast a battery charges or discharges relative to its capacity: 1C means full capacity delivered in one hour, 0.5C in two hours. It links a battery's energy (kWh) to its continuous power (kW) capability.
Cycle life: The number of charge-discharge cycles completed before capacity reaches a stated test threshold. It depends on the test method, chemistry, depth of discharge, temperature and operating limits; compare the warranty's years, throughput and retained-capacity terms as well as any cycle figure.
Depth of Discharge (DoD): The percentage of a battery's total capacity that is drawn down from full. The maximum recommended DoD sets how much of the rated capacity is actually usable: e.g. a 10 kWh battery rated to 90% DoD gives 9 kWh. Shallower discharges generally extend cycle life.
LiFePO4 (LFP): Lithium iron phosphate: a lithium-ion cathode chemistry widely used in stationary storage. Chemistry changes cell-level thermal and energy-density trade-offs, but it does not by itself establish a finished battery system's safety, service life or suitability.
Round-trip efficiency: The share of energy put into a battery system that can be delivered again after charging and discharging losses, expressed as a percentage. Compare exact products on the same test boundary because some figures include power-conversion losses and others do not.
State of Charge (SoC): An estimate of the charge currently available in a battery, usually expressed as a percentage under the battery-management system's defined operating window. The displayed value and usable reserve depend on the product's controls and settings.
Usable capacity: The energy a battery actually delivers in normal use, in kWh: its total (nominal) capacity multiplied by the allowed depth of discharge. This, not the nameplate capacity, is the number that matters for sizing how much of your home it can run.

Electrical

Maximum power point (Vmp / Imp): The voltage (Vmp) and current (Imp) at which a panel delivers its greatest power under given conditions; their product is the rated power. MPPT keeps the array operating at this point as conditions shift.
Maximum series fuse rating: The largest overcurrent-protection fuse rating a panel is rated to tolerate, used to protect parallel strings from reverse fault current. It is a key input when an installer designs string combiner and fusing.
Maximum system voltage: The highest DC system voltage for which a module is rated under its stated certification. It is one limit in string design; the calculation must also respect the inverter, connected equipment and applicable installation requirements under cold conditions.
MC4 connector: A named family of locking DC connectors used in PV wiring. Similar appearance does not establish mating compatibility: follow the exact connector manufacturer, type, tooling and installation requirements recorded for the system.
Open-circuit voltage (Voc): The voltage measured across a PV module or string with its circuit open under the stated conditions. Because voltage changes with cell temperature, string design applies the exact Voc coefficient and site design temperature against every applicable equipment and system limit.
Rapid shutdown: A safety function designed to reduce voltage in specified PV conductors when triggered. Requirements and permitted implementations vary by jurisdiction and system, so the term is not a substitute for the applicable rules and equipment instructions.
Short-circuit current (Isc): The current measured when a PV device's output terminals are short-circuited under stated conditions. Design current checks apply the exact module value and the factors required for irradiance, bifacial contribution and the applicable rules.

System & Install

Glass-glass (dual-glass) module: A PV module constructed with glass on both front and rear instead of a polymer backsheet. The construction may be used for monofacial or bifacial products; mass, load rating, mounting, electrical data and warranty remain exact-model questions.
IEC 61215: The international design-qualification and type-approval series for terrestrial PV modules. It specifies qualification requirements and test procedures; passing it is not a performance ranking or proof that a module suits a particular site.
IEC 61730: The international PV-module safety-qualification series. Part 1 covers construction requirements and Part 2 covers testing; qualification is not a substitute for current product-list status or a site-specific compliant design.
IP rating: Ingress Protection rating: an IEC classification for an enclosure's protection against access, solid objects and water. The two characteristic numerals address different hazards; read the exact rating and test conditions rather than translating it into a general outdoor-suitability claim.
Mechanical load rating: A declared pressure a module has been qualified against under stated test and mounting conditions, commonly reported separately for front and rear loading. Use the exact installation manual, approved mounting zones and site structural design rather than the headline pressure alone.
Tilt and azimuth: Tilt is the angle of a panel from horizontal; azimuth is the compass direction it faces. Together with location, shading and horizon conditions they shape when and how much sunlight the array receives. In Australia, north-facing exposure often favours annual yield when other constraints are equal.

Performance

Annual degradation: A stated or measured year-to-year change in module output. Manufacturer warranty curves may separate an initial allowance from later annual allowances; use the exact product document and its conditions rather than a generic technology rate.
Capacity factor: The ratio of actual energy output over a period to the energy that would have been produced at the selected rated capacity for every hour in that period. State whether the denominator uses DC or AC capacity when comparing PV results.
Performance (power) warranty: A manufacturer's promise about a module's output floor over a stated period and under stated conditions. It is separate from the product warranty, and its exclusions, test method and remedy matter as much as the headline duration.
Performance ratio (PR): A normalised PV-system performance metric, commonly calculated as final yield divided by reference yield under a declared method. It reflects multiple system and environmental effects, so the calculation basis must be stated before two results are compared.
Product (workmanship) warranty: The warranty covering defined defects in the module itself, distinct from the performance warranty. Read the exact coverage, exclusions, claim process and responsibility for labour, freight, removal and reinstallation.
Soiling loss: The output lost when dust, dirt, pollen, bird droppings, or snow accumulate on panel glass and block light. It varies by climate and tilt and is recovered by rain or cleaning; it is one of the loss terms a performance ratio captures.
Specific yield: Annual energy produced per unit of installed capacity, in kWh per kWp per year. It normalises output by system size, making it the fair way to compare productivity across systems and locations.

Grid & Market

Curtailment: The deliberate reduction of a system's output below the power otherwise available, including inverter control used to meet an export limit. The cause and control authority must be established from the site's settings, connection requirements and event evidence.
Feed-in tariff (FiT): The rate an electricity retailer or programme pays or credits for eligible solar energy exported to the grid, usually stated in cents per kilowatt-hour. The current rate and conditions come from the live electricity plan.
Frequency Control Ancillary Services (FCAS): Services procured through Australia's National Electricity Market to help keep power-system frequency within the required operating bands. Different FCAS markets have distinct response and enablement requirements; battery participation depends on registration, control and market arrangements.
Net metering: A metering arrangement that records the balance between electricity imported from and exported to the grid under the applicable billing rules. It is distinct from gross metering, which records all generation as export.
Self-consumption: Solar electricity used within the property rather than exported to the grid. Its bill value depends on when it offsets imports and on the current retail and feed-in tariff conditions.
Time-of-use (TOU) tariff: An electricity tariff with different rates for defined time periods such as peak, shoulder and off-peak. Assess solar, battery and load-shifting value against the current plan's actual periods, rates and conditions.
Virtual power plant (VPP): A group of batteries and sometimes other customer devices coordinated through software and communications. Eligibility, control rights, reserve behaviour and customer benefit are defined by the live programme and contract.

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