Solar Engineering
71 terms in the solar engineering category.
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Albedo
Albedo is the fraction of sunlight a surface reflects, expressed from 0 (absorbs everything) to 1 (reflects everything). PVsyst uses 0.20 as its default ground albedo, while fresh snow can exceed 0.8. Designers use measured or modelled albedo to estimate rear-side irradiance and bifacial gain in energy yield studies.
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Arc Fault
A solar arc fault is an unintended electric arc in PV DC wiring, usually from loose connectors, damaged insulation or corroded terminals. NEC 690.11 requires listed DC arc-fault circuit protection (AFCI, tested to UL 1699B) for PV systems with DC circuits at 80 V or more. Designers usually meet it with inverters that include built-in DC AFCI.
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Azimuth
Azimuth is the compass direction a PV array faces. Most conventions measure it clockwise from true north, so south is 180 degrees, though PVsyst uses 0 degrees for south. In the northern hemisphere south-facing arrays maximise annual energy, and designers model azimuth alongside tilt to estimate yield on each roof face.
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Backtracking
Backtracking is a single-axis tracker control strategy that rotates modules away from the ideal sun angle in early morning and late afternoon so one row does not shade the next. The controller uses row pitch, module width and terrain slope, which together set the ground coverage ratio (GCR), to choose the angle.
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Bifacial Gain
Bifacial gain is the extra energy a bifacial PV module produces from light reaching its rear side, expressed as a percentage over a comparable monofacial module. IEC TS 60904-1-2:2019 defines how bifaciality is measured. Ground albedo, mounting height, row spacing and tracking all affect the gain modelled in energy yield studies.
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Bypass Diode Activation
Bypass diode activation is the moment a PV module's bypass diode starts conducting because a shaded or damaged cell group is pushed into reverse bias. Current flows around that cell group, limiting hot-spot heating, but the module loses that substring's output and the I-V curve gains a step. Designers model it in shading studies.
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Cell Microcrack
A cell microcrack is a fine fracture in a silicon solar cell, often invisible to the eye, caused by handling, transport, installation, wind, snow or thermal stress. Electroluminescence (EL) imaging, described in IEC TS 60904-13:2018, is the standard way to find them. Cracks can grow over time and cause power loss.
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Combiner Box
A combiner box is an electrical enclosure that merges several PV source circuits into one DC output feeding the inverter. Each string input normally has its own fuse, sized under NEC 690.9, and the box may also hold a disconnect and surge protection. Designers use combiners on commercial and utility arrays with many parallel strings.
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Conductor Ampacity
Conductor ampacity is the maximum current, in amperes, a conductor can carry continuously without exceeding its insulation temperature rating. NEC 2023 Table 310.16 lists base values for 60°C, 75°C and 90°C insulation. Solar designers then apply temperature and conduit-fill corrections and size PV circuits using the NEC 690.8 current rules.
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Conduit Fill
Conduit fill is the share of a raceway's internal cross-section that conductors may occupy. NEC Chapter 9, Table 1 limits fill to 53% for one conductor, 31% for two and 40% for three or more. Solar designers check fill on every PV homerun because more current-carrying conductors also force ampacity derating.
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DC Optimizer
A DC optimizer is a module-level power electronic device that performs maximum power point tracking for each PV module while feeding a central string inverter. Products from SolarEdge and Tigo are common examples. Designers use optimizers to reduce mismatch and shading losses and to meet NEC 690.12 rapid shutdown requirements.
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DHI (Diffuse Horizontal Irradiance)
DHI (Diffuse Horizontal Irradiance) is the solar radiation reaching a horizontal surface after scattering by the atmosphere, clouds and aerosols, excluding the direct beam. It is measured in W/m² with a shaded pyranometer, classified under ISO 9060:2018. Solar designers combine DHI with DNI to derive GHI and model transposed irradiance.
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Diffuse Irradiance
Diffuse irradiance is the part of solar radiation scattered by air molecules, clouds and aerosols that arrives from the whole sky rather than straight from the sun. Measured on a horizontal plane it is called DHI, recorded with a shaded pyranometer under ISO 9060:2018. It strongly influences yield in cloudy or hazy climates.
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DNI (Direct Normal Irradiance)
DNI (Direct Normal Irradiance) is the solar radiation received per unit area on a surface held perpendicular to the sun's direct beam, excluding diffuse and reflected light. It is measured with a sun-tracking pyrheliometer classified under ISO 9060:2018. DNI is a key input for tracker yield modeling and concentrated solar power.
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Dual-Axis Tracker
A dual-axis tracker rotates PV modules on two axes, following the sun's daily east-west path and its seasonal elevation change, so modules stay close to perpendicular to the direct beam. Tracker design qualification is covered by IEC 62817:2014. Dual-axis units are mainly used for concentrating PV and small high-yield installations.
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East-West Layout
An east-west layout mounts PV modules in alternating east-facing and west-facing rows, often at low tilt, so the array produces a flatter daily profile than a south-facing layout. It fits more modules on flat commercial roofs and suits California's Net Billing Tariff (NEM 3.0), effective April 2023, which rewards self-consumption.
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EGC (Equipment Grounding Conductor)
An EGC (equipment grounding conductor) bonds exposed metal parts such as module frames, racking, enclosures and conduit back to the source, giving fault current a low-impedance path so the overcurrent device trips. NEC 250.122 sizes the EGC from the rating of the upstream OCPD, and NEC 2023 Section 690.45 applies that same table to PV circuits.
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EL Imaging (Electroluminescence)
EL (electroluminescence) imaging photographs PV cells while current is driven through them, usually in darkness, so they emit near-infrared light that reveals microcracks, broken fingers, inactive areas and soldering defects. IEC TS 60904-13:2018 describes the method. Manufacturers, inspectors and asset owners use EL for factory quality control and field audits.
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Equipotential Bonding
Equipotential bonding connects all exposed metal parts of a PV system, including module frames, racking, enclosures and raceways, so they stay at the same electrical potential. In the US, NEC 690.43 and Article 250 govern PV bonding, and IEC 60364-7-712 covers it internationally. It reduces shock risk during faults and limits surge damage.
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Fill Factor
Fill factor (FF) is the ratio of a solar cell's or module's maximum power to the product of its open-circuit voltage and short-circuit current: FF = Pmax / (Voc x Isc). It is measured from I-V curves under standard test conditions defined in IEC 60904-1. Higher values indicate lower resistive losses and better cell quality.
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Fixed Tilt
Fixed-tilt mounting holds PV modules at a constant tilt angle and azimuth that do not change during the day or year. The angle is usually chosen near the site latitude for annual yield or adjusted for land, wind or seasonal goals. Structural design in the US follows ASCE 7 wind and snow loads.
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GHI (Global Horizontal Irradiance)
GHI stands for Global Horizontal Irradiance, the total sunlight falling on a flat, horizontal surface, measured in W/m² or summed as kWh/m² per day or year. It equals direct normal irradiance multiplied by the cosine of the solar zenith angle, plus diffuse horizontal irradiance. GHI is the starting input for every solar yield model.
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Ground Fault
A solar ground fault is an unintended conductive path between an energized PV circuit and earth or bonded metal, often caused by damaged insulation, water ingress or pinched wiring. NEC 690.41(B) requires ground-fault protection for most DC PV arrays. An inverter alarm signals a condition to investigate; it does not pinpoint the failed component.
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Horizon Profile
Horizon profile is the line of elevation angles, plotted against azimuth, formed by distant hills, ridgelines and buildings around a solar site. PVsyst imports it in its Horizon tool, often from PVGIS or Meteonorm data, to calculate far-shading loss, which designers keep separate from near shading in yield reports.
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Hot Spot
A hot spot is localized overheating in a PV cell that occurs when a shaded, cracked or mismatched cell is forced into reverse bias and dissipates string current as heat. IEC 61215-2:2021 includes a hot-spot endurance test (MQT 09), and designers rely on bypass diodes and good layout to limit the risk.
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I-V Curve
The I-V curve is a graph of a PV module or string's current against voltage under given irradiance and temperature, running from short-circuit current to open-circuit voltage. IEC 60904-1 defines how to measure it, and installers and O&M teams use I-V curve tracers to verify performance and diagnose faults.
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Imp (Maximum Power Current)
Imp (maximum power current) is the current a PV module delivers at its maximum power point, measured at Standard Test Conditions and printed on the datasheet. Multiplying Imp by Vmp gives the rated power, Pmax. Imp is usually about 92 to 95% of Isc, and designers use it to check that parallel strings fit each inverter MPPT input.
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IR Thermography
IR thermography is the use of infrared cameras to find temperature anomalies in operating PV modules and electrical connections, such as hot spots, failed bypass diodes and loose connectors. IEC TS 62446-3:2017 sets the method for outdoor PV thermography, used by O&M teams and asset owners during commissioning and inspections.
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Isc (Short-Circuit Current)
Isc (short-circuit current) is the current a PV module delivers when its output terminals are shorted together, measured at Standard Test Conditions. Under NEC 2023 Section 690.8, maximum PV circuit current equals 1.25 times summed Isc, and designers use that figure to size conductors, fuses and other overcurrent devices.
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LeTID
LeTID (Light and elevated Temperature Induced Degradation) is a power loss in crystalline silicon modules, especially PERC, that develops over months to years under sunlight and cell temperatures above about 50°C. IEC TS 63342:2022 defines a test for it, and designers factor LeTID susceptibility into degradation assumptions and module selection.
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LID (Light Induced Degradation)
LID (Light Induced Degradation) is the initial drop in power that crystalline silicon PV modules show during their first hours to weeks of sunlight, mainly from boron-oxygen defects in p-type cells. IEC 61215-2:2021 includes a stabilization procedure for it, and designers enter first-year LID as a loss in PVsyst.
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Lightning Protection System (LPS)
A lightning protection system (LPS) is the set of air terminals, down conductors and earth electrodes that intercepts direct lightning strikes and carries the current safely to ground. The IEC 62305 series, including the IEC 62305-2 risk assessment, guides LPS design for solar plants alongside surge protective devices.
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MLPE (Module-Level Power Electronics)
MLPE (Module-Level Power Electronics) is the group of devices mounted at each PV module, mainly microinverters and DC optimizers, that control or convert power panel by panel. Since NEC 2017, Section 690.12 rapid shutdown has made MLPE the most common compliance route on US rooftops. Designers also use MLPE to reduce shading losses and monitor each panel.
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Module Efficiency
Module efficiency is the share of incident sunlight a PV module converts into electrical power, measured at Standard Test Conditions (1000 W/m², 25°C cell temperature, AM1.5 spectrum) per IEC 61215. It equals rated power divided by irradiance times module area. Designers use it to estimate how many kilowatts fit on a roof or plot.
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Module Frame Bonding
Module frame bonding is the electrical connection of every PV module's metal frame to the equipment grounding conductor so a fault cannot leave the frame energised. In the United States it is required by NEC 690.43 and is usually achieved with UL 2703 listed racking, lay-in lugs, or bonding washers that inspectors verify during plan review.
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Module Mismatch
Module mismatch is the energy loss that occurs when PV modules in the same series string have slightly different electrical characteristics, so the string current is limited by its weaker modules. Simulation tools such as PVsyst apply a default mismatch loss, commonly around 1-2%, and designers reduce it through careful binning, string layout, or module-level electronics.
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Module Power Tolerance
Module power tolerance is the range a manufacturer guarantees around a PV module's nameplate Pmax at Standard Test Conditions, defined under IEC 61215 testing. Most current datasheets state a positive-only tolerance such as 0 to +5 W, meaning no module ships below nameplate. Designers check it because tighter tolerance reduces string mismatch loss.
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MPPT (Maximum Power Point Tracking)
MPPT (Maximum Power Point Tracking) is the control function inside a solar inverter or charge controller that keeps adjusting array voltage to draw the most available power as irradiance and temperature change. Its efficiency is tested under EN 50530. Designers count MPPT inputs to decide how strings with different orientations or lengths are connected.
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NOCT (Nominal Operating Cell Temperature)
NOCT stands for Nominal Operating Cell Temperature, the temperature a PV module's cells reach at 800 W/m² irradiance, 20°C ambient air and 1 m/s wind on an open rack. Most crystalline modules rate between about 41°C and 46°C. IEC 61215:2016 replaced NOCT with NMOT, which is measured with the module under electrical load.
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OCPD (Overcurrent Protection Device)
An OCPD (overcurrent protective device) is a fuse or circuit breaker that opens a circuit when current exceeds a safe level, protecting conductors and equipment. Under NEC 2023 Section 690.9(B), a PV circuit OCPD is rated at least 125% of the maximum circuit current, itself 125% of Isc, so roughly 1.56 x Isc, rounded up to a standard size.
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Performance Ratio (PR)
Performance ratio (PR) in solar is a PV system's final yield divided by its reference yield, written PR = E_AC ÷ (P_STC × H_POA ÷ 1 kW/m²) in IEC 61724-1:2021. It shows what share of the energy available from plane-of-array sunlight the plant actually delivers, so O&M teams, lenders and auditors use it to track losses.
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PID (Potential Induced Degradation)
PID (Potential Induced Degradation) is power loss in PV modules caused by leakage current driven by high voltage between the cells and the grounded frame. Modules are screened for it with the IEC TS 62804-1 test. Designers manage PID risk through module selection, string voltage, inverter grounding and PID-recovery functions, especially in hot, humid sites.
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POA Irradiance (Plane of Array)
POA stands for Plane of Array, and POA irradiance is the total sunlight reaching the tilted surface of a PV module, measured in W/m². It adds direct beam, sky diffuse and ground-reflected light, usually calculated from GHI with a transposition model such as Perez (1990). Solar designers use POA irradiance as the main input for energy yield.
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PTC (PVUSA Test Conditions)
A PTC rating is a solar module's power output at PVUSA Test Conditions: 1000 W/m² irradiance, 20°C ambient air, 1 m/s wind and AM 1.5 spectrum. The California Energy Commission lists PTC values for modules. A PTC rating usually sits below the STC nameplate, so designers use it to estimate realistic field output.
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PV Wire vs USE-2
PV Wire vs USE-2 compares two conductor types used in solar DC circuits. PV Wire, listed to UL 4703, is a sunlight-resistant single conductor built for exposed module and array wiring. USE-2, listed to UL 854, is designed for underground and wet locations. NEC 690.31 sets where each may be used in PV source circuits.
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Rapid Shutdown
Rapid shutdown is the NEC 690.12 requirement to de-energize PV conductors on buildings so firefighters can work safely. Since NEC 2017, conductors outside the array boundary (1 ft from the array) must drop to 30 V within 30 seconds, and conductors inside it to 80 V, usually met with module-level power electronics or a UL 3741 system.
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Recombiner Box
A recombiner box is a second-stage DC combiner used in utility-scale central-inverter solar plants. It merges the outputs of several string combiner boxes into fewer larger feeders before the inverter, with fusing, a disconnect and often current monitoring. Equipment is commonly rated 1,500 V DC, and NEC 690.9 governs its overcurrent protection.
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Reverse Current
Reverse current in a PV array is current flowing backwards through a module or string, usually when a fault or short lowers one string's voltage and parallel strings feed into it. Modules are tested for it by the reverse current overload test in IEC 61730-2, and NEC 690.9 requires overcurrent protection sized to prevent damage.
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Series Fuse Rating
Series fuse rating, or maximum series fuse, is the largest overcurrent protective device a PV module can safely have in series, as listed on its datasheet. It is verified by the reverse current overload test in IEC 61730-2. Designers use it under NEC 690.9 to size string fuses in combiner boxes for parallel strings.
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Shading Analysis
Shading analysis is the process of calculating how trees, buildings, rooftop equipment and neighbouring rows reduce a solar array's energy output. It combines sun-path geometry, a 3D site model and time-step simulation, often reported as TSRF (Total Solar Resource Fraction). Designers use it to place modules, choose MLPE and support permit and yield reports.
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Single-Axis Tracker
A single-axis tracker is ground-mount racking that rotates modules around one axis, usually north-south, to follow the sun from east to west each day. Trackers are design-qualified under IEC 62817:2014 and use backtracking to limit row-to-row shading. Utility-scale developers choose them because they raise energy yield over fixed-tilt arrays.
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SLD (Single Line Diagram)
An SLD (Single Line Diagram) is a simplified electrical drawing that shows a solar PV system with one line per circuit, from modules and strings through inverters, disconnects and protection to the point of interconnection. US permit sets follow NEC 2023 Article 690, and Indian CEIG submissions require one. AHJs, utilities and inspectors review it first.
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Soiling Loss
Soiling loss is the drop in PV energy output caused by dust, pollen, bird droppings and other deposits on module glass. IEC 61724-1:2021 defines how monitoring systems measure it as a soiling ratio. Designers enter a site-specific soiling assumption in yield models and plan cleaning schedules, especially in dusty regions such as Rajasthan or the Middle East.
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Solar Pathfinder
Solar Pathfinder is a handheld shade-measurement instrument that uses a transparent reflective dome over a latitude-specific sun-path diagram. Obstructions reflected on the dome are traced to estimate monthly and annual shading at a proposed array location. Installers and site surveyors have used it for decades, though LiDAR and digital tools now dominate.
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Solar Tracker
A solar tracker is a mounting system that rotates PV modules during the day to follow the sun, increasing the irradiance they capture. Single-axis trackers turn on one axis and dual-axis trackers on two. Trackers are design-qualified under IEC 62817:2014, and utility-scale developers use them to raise energy yield per MW installed.
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Solar Window
The solar window is the part of each day and year when the sun is high enough to deliver useful irradiance to a PV array. Shade-analysis tools such as the Solar Pathfinder commonly treat 9 am to 3 pm solar time as the core window, and designers check that it stays free of obstructions.
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SPD (Surge Protection Device)
A surge protective device (SPD) is a component that limits transient overvoltages from lightning or switching by diverting surge current to earth. PV systems use DC SPDs tested to IEC 61643-31 on array and inverter inputs, and AC SPDs to IEC 61643-11 or UL 1449, with Type 1, 2, or 3 ratings by location.
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Specific Yield
Specific yield is the annual AC energy a PV system produces divided by its DC nameplate capacity, expressed in kWh/kWp per year. IEC 61724-1 defines the related final yield metric. Designers and investors use it to compare sites and system designs of different sizes; well-designed Indian plants often achieve roughly 1,400 to 1,700 kWh/kWp.
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STC (Standard Test Conditions)
STC stands for Standard Test Conditions, the laboratory reference used to rate every PV module: 1000 W/m² irradiance, 25°C cell temperature and the AM 1.5 spectrum defined in IEC 60904-3. The wattage, Voc, Isc, Vmp and Imp printed on a module label are STC values, which lets designers compare modules on a common basis.
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String Current
String current is the current flowing through a series-connected PV string, which is the same through every module in that string. Under NEC 2023 690.8(A)(1), maximum circuit current is 1.25 times the summed module short-circuit current (Isc), and designers use it to size conductors, fuses, and inverter MPPT inputs.
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String Sizing
String sizing is the calculation of how many PV modules to wire in series in each string. The maximum count keeps cold-weather open-circuit voltage below the inverter and NEC 2023 690.7 limits, and the minimum count keeps hot-weather maximum power voltage above the inverter's MPPT lower bound, so the inverter operates all year.
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String Voltage
String voltage is the sum of the voltages of all PV modules wired in series in one string. Cold-weather Voc sets the maximum and must stay under the inverter rating and NEC 2023 690.7, which caps one- and two-family dwellings at 600 V, while hot-weather Vmp must stay within the inverter MPPT range.
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Stringing
Stringing is the design and wiring of PV modules into series strings and assigning those strings to inverter MPPT inputs. A stringing plan sets module counts, routes, connectors, and orientation groups so voltage and current stay within limits; IEC 62548 sets the international design requirements for PV arrays and their wiring.
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Temperature Coefficient
A temperature coefficient is the rate at which a PV module parameter changes with cell temperature, usually expressed in %/°C relative to the 25 °C rating. Values are measured under IEC 60891 and IEC 60904 procedures; typical crystalline modules lose about 0.29 to 0.40% of Pmax per °C, which matters for hot-climate yield and string sizing.
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Three-Line Diagram
A three-line diagram is an electrical drawing that shows each phase conductor, the neutral, and grounding separately, rather than one line for the whole circuit. Utilities often request it for larger commercial or utility-scale PV interconnections to review protection, CT and PT connections, and relay settings under IEEE 1547-2018 and local grid codes.
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Tilt Angle
Tilt angle is the angle between a PV module's surface and the horizontal, measured in degrees. For fixed systems the annual energy optimum is often close to the site's latitude, roughly 10 to 30 degrees across most of India, and designers adjust it for roof pitch, wind loading, row spacing, and seasonal demand.
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Vmp (Maximum Power Voltage)
Vmp (maximum power voltage, also written V_MPP) is the voltage at which a PV module delivers its peak power, listed on datasheets at Standard Test Conditions of 1000 W/m², 25 °C cell temperature, AM1.5 (IEC 60904-3). It is typically about 80 to 85 percent of Voc, and designers use hot-weather Vmp to keep strings above the inverter MPPT minimum.
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Voc (Open-Circuit Voltage)
Voc (open-circuit voltage) is the voltage across a PV module's terminals when no current flows, the highest voltage it produces. It is rated at Standard Test Conditions of 1,000 W/m2, 25 °C cell temperature, and AM 1.5 spectrum per IEC 60904, and it controls maximum string length under NEC 2023 690.7.
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Voltage Drop
Voltage drop is the reduction in voltage along a conductor caused by its resistance and the current it carries. NEC informational notes suggest limiting branch circuits to 3% and feeders plus branch circuits to 5%; solar designers often target lower DC and AC drop to reduce energy losses and keep inverters within their operating range.
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What is BOQ in a Solar Project?
BOQ stands for bill of quantities, an itemised schedule listing every material, equipment item and work activity in a solar project with its unit and measured quantity. EPCs and developers build it from the layout, single line diagram and structural drawings, then use it to price tenders, plan procurement and bill measured progress.
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Wire Sizing
Wire sizing is selecting a conductor's material, insulation, and cross-section so it carries the circuit's current safely with acceptable voltage drop. For US PV source circuits, NEC 2023 690.8 typically applies 125% of Isc and another 125% continuous-duty factor, a combined 156%, before ampacity corrections from Table 310.16.
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