NABCEP PVDS key terms

A laboratory reference set of 1000 W/m² irradiance, 25 °C cell temperature, and AM 1.5 spectrum used to rate module power. Nameplate ratings on datasheets are STC values, not expected field output.

The instantaneous solar power density striking a surface, expressed in watts per square meter. It drives array current and varies with sun angle, clouds, haze, and shading through the day.

Irradiance measured on the tilted surface of the modules rather than on a horizontal plane. POA combines direct beam, diffuse sky, and ground-reflected components and is the input for array output estimates.

The percentage change in module maximum power per degree Celsius of cell temperature, usually negative. A typical value near −0.35 %/°C means hot days reduce output below nameplate.

The change in module open-circuit or maximum-power voltage per degree Celsius. Cold temperatures raise voltage, so this coefficient sets the worst-case maximum string voltage used for conductor and equipment checks.

The highest voltage a module or system component is rated to withstand, commonly 1000 V or 1500 V. String voltage corrected to record low temperature must stay below this rating.

Module voltage with no load connected, the highest voltage the module produces at a given temperature and irradiance. String Voc corrected for cold temperature is used for voltage-limit calculations.

Current that flows when module output terminals are shorted at a given irradiance. It sets the basis for conductor ampacity and overcurrent device sizing after applying the required 1.25 factor.

The operating voltage and current pair where a module or array produces its greatest power on the current-voltage curve. Inverters continuously adjust operating point to track this value.

Inverter control that continuously sweeps array operating voltage to hold the array at its maximum power point as irradiance and temperature shift. MPPT window and count constrain string design.

Selecting the number of modules per series string so voltage stays inside the inverter MPPT window across expected temperatures and never exceeds maximum system voltage at record cold.

Array nameplate DC power divided by inverter rated AC output. Values above one accept some clipping at peak sun in exchange for better production during off-peak hours.

Energy lost when array DC power exceeds the inverter's AC limit and the excess cannot be converted. It occurs near midday peaks and is a deliberate tradeoff in high DC-to-AC ratio designs.

The maximum continuous current a conductor can carry under its installation conditions without exceeding its insulation temperature rating. Ampacity is adjusted for ambient temperature and conduit fill.

The 1.25 multiplier applied to continuous currents such as PV source and output circuits before sizing conductors and overcurrent devices. It accounts for sustained operation at full output for three hours or more.

Loss of voltage along conductors caused by resistance, reducing power delivered to loads. Designers commonly target a few percent on DC and AC circuits to preserve system yield.

A fuse or circuit breaker that interrupts excessive current to protect conductors and equipment. PV source circuits require careful selection because modules can backfeed fault current.

The maximum overcurrent device rating permitted in series with a module or string, stated on the datasheet. Exceeding it risks module damage from reverse current in a faulted parallel string.

Detection and interruption of unintended current paths from PV conductors to ground. Required protection types and thresholds differ for grounded and ungrounded systems.

Detection of serial or parallel arcing in DC circuits and rapid shutdown of the affected circuit. Inverters commonly provide this function as an integrated listed feature.

A requirement that conductors near a roof or array be reduced to a safe voltage within a specified time after activation. It protects firefighters and service personnel from energized DC.

Ratio of AC energy delivered to DC energy consumed, including conversion and standby losses. Weighted efficiency across power levels is more useful than a single peak figure.

The manufacturer's published power, voltage, and current values under defined test conditions. It is the starting point for calculations, not a prediction of actual site production.

The angle between the module surface and horizontal. Tilt affects annual and seasonal energy capture, snow shedding, and row-to-row shading spacing.

Compass direction a module surface faces, usually measured in degrees from true north. Azimuth and tilt together define array orientation and expected irradiance profile.

Center-to-center or edge-to-edge distance between module rows, set so winter sun clears the front row. Tighter spacing raises energy density but increases inter-row shading losses.

Study of obstructions, terrain, and neighboring rows that block sunlight at specific times. Results guide array layout, tilt, and string routing to limit mismatch losses.

A diode across a module substring that provides a current path around shaded or faulty cells, limiting hot spots and power loss. Bypass diodes shape how partial shading affects string output.

Localized overheating where a cell or connection limits current while the rest of the string drives current through it. Persistent hot spots degrade encapsulant and can cause module failure.

Force exerted on modules, racking, and roof attachments by wind pressure, calculated from basic wind speed, exposure category, height, and topography. It governs attachment spacing and ballast weight.

Downward force from accumulated snow plus related drift and rain-on-snow surcharge. It sets module and racking strength requirements and influences tilt for shedding.

The connection between racking and the building structure, such as lag screws into rafters or clamps to standing seams. Attachment type and spacing must transfer wind and snow loads safely.

Methods and flashing used to keep water out where mounting hardware passes through roofing. Correct flashing detail is a primary warranty and water-intrusion concern.

Rack that holds modules down with weight instead of penetrations, common on low-slope commercial roofs. Ballast design depends on wind uplift, roof dead-load capacity, and membrane protection.

Fraction of incoming sunlight reflected by a surface, such as grass, gravel, or white membrane. Higher albedo raises ground-reflected irradiance on tilted arrays.

The moment the sun crosses the local meridian and reaches its highest altitude for that day. Shading studies and row-spacing checks are typically evaluated around this time.

The degree to which a site receives unobstructed sunlight, considering obstructions, trees, and future construction. It is documented during site assessment and may be protected by local ordinances.

On-site collection of measurements and observations such as roof dimensions, slope, obstructions, structural members, and electrical service details. It forms the factual basis for design.

The location where the PV system electrically connects to the utility or premises wiring, such as a load-side breaker or supply-side tap. It determines applicable sizing and protection rules.

The busbar and main breaker current ratings of the existing electrical panel. The 120 % rule limits how much PV breaker capacity can be added without a panel upgrade.

A billing arrangement crediting exported energy against consumption, often at the retail rate. Its rules affect system sizing and the value of exported production.

Modeled annual or monthly energy output from irradiance, system losses, and equipment performance. It supports financial analysis and is typically validated against a performance ratio.

Actual energy yield divided by the theoretical yield at reference irradiance, factoring temperature and system losses. It allows comparison of systems of different sizes and locations.

A drawing showing electrical components and connections with one line per circuit. It documents conductors, devices, grounding, and ratings for permitting and inspection.

Submission set including drawings, equipment datasheets, structural calculations, and electrical calculations required by the authority having jurisdiction. Completeness drives approval timelines.

The official body that reviews and approves plans, issues permits, and inspects installations for code compliance. Local amendments can exceed the base electrical code.

Required placards and markings identifying circuits, disconnects, voltage, and shutdown information. Missing or incorrect labels are frequent inspection corrections.

A switch that opens the DC circuit between array and inverter for service. Its rating must handle DC arc interruption and its location must be accessible to responders.

Conductor bonding non-current-carrying metal parts to ground to clear faults and limit touch voltage. Its size follows code tables based on the overcurrent device rating.

Procedure that de-energizes, isolates, and marks equipment before work begins. It prevents unexpected energization during installation or maintenance.

Arc-rated clothing, gloves, eye protection, and fall protection worn for identified hazards. Selection follows a documented hazard assessment and shock risk category.

Systems such as harnesses, anchors, and guardrails that arrest a fall from elevation. Roof work triggers requirements at defined heights and requires trained, competent oversight.