NABCEP SW Associate flashcards

32 cards. Read the term and the definition here. The deck at the top of this page is optional.

Standard Test Conditions

Irradiance 1000 W/m², cell temperature 25 °C, and AM 1.5 spectrum. Module nameplate power, voltage, and current are rated at STC, so real-world output is usually lower.

NOCT

Nominal Operating Cell Temperature measured at 800 W/m², 20 °C ambient, and 1 m/s wind. Typical framed modules reach 42-48 °C, used to estimate real cell temperature.

Peak sun hours

Number of hours per day at 1000 W/m² that would deliver the same daily irradiation. A 5 kW array in a 5 PSH location yields about 25 kWh/day before derating.

Performance ratio

Ratio of actual AC energy to the theoretical energy from STC array rating times plane-of-array irradiation. It captures temperature, soiling, wiring, and inverter losses; 0.75-0.85 is typical.

Bypass diode function

A diode across a module substring that provides an alternate current path when cells are shaded or damaged. It limits power loss and prevents reverse-bias heating of shaded cells.

Temperature coefficient of power

Percent change in module power per degree Celsius of cell temperature, typically −0.3 to −0.45 %/°C for crystalline silicon. Hot cells therefore reduce output below STC ratings.

Fixed tilt orientation

In the northern hemisphere, modules face true south at a tilt near site latitude. Steeper tilt boosts winter production, shallower tilt boosts summer production.

Irradiance vs insolation

Irradiance is instantaneous power per area in W/m². Insolation is energy per area over a period, usually kWh/m²/day, and determines daily energy yield.

Ohm's law and power

V = I × R and P = V × I, which also gives P = I²R and P = V²/R. These relationships underpin voltage drop, conductor sizing, and load calculations.

Series string behavior

Modules in series add voltages while current stays the same. String current is limited by the lowest-current module, so mismatched or shaded modules reduce total output.

Parallel connection behavior

Parallel-connected sources add currents while voltage stays the same. Mismatched parallel strings are limited by the lowest-voltage string and can cause circulating current.

1.25 continuous factor

NEC requires multiplying PV source and output currents by 1.25 for conductor ampacity and overcurrent sizing, treating irradiance-enhanced current as a continuous load.

DC voltage drop formula

Vdrop = 2 × L × I × R, where L is one-way length and R is conductor resistance per unit length. The factor two accounts for the round-trip path.

Energy unit conversions

1 kWh equals 1000 Wh and 3.6 MJ. One horsepower is about 0.746 kW. These conversions link utility bills, motor loads, and array sizing.

Three-phase AC power

P = √3 × Vline × Iline × power factor for a balanced three-phase load. Single-phase power is simply V × I × power factor.

Fill factor

Ratio of maximum power to the product Voc × Isc. A higher fill factor means a squarer I-V curve and a more efficient module, typically 0.7-0.8 for silicon.

Maximum power point

The point on the I-V curve where Vmp × Imp is greatest. MPPT electronics continuously adjust operating voltage to track this point as irradiance and temperature change.

Cold-temperature Voc check

At the lowest expected ambient temperature, string open-circuit voltage rises and must remain below the inverter maximum DC input voltage, or the inverter can be damaged.

Hot-temperature Vmp check

At the highest expected cell temperature, string maximum power voltage falls and must stay above the inverter MPPT minimum, or the inverter drops out of tracking range.

DC/AC ratio

Array STC power divided by inverter AC rating. Ratios of 1.1-1.3 are common; when array output exceeds the inverter rating, the inverter clips AC power.

Depth of discharge

Fraction of battery capacity withdrawn in a cycle. Lead-acid is typically limited to 50 % for life, while lithium can often use 80-100 % of rated capacity.

MPPT vs PWM controller

MPPT controllers track the array maximum power point and allow higher array voltage, gaining output in cool weather. PWM controllers clamp array voltage to battery voltage and are simpler.

Rapid shutdown

NEC 690.12 requirement that conductors within 1 ft of the array be limited to 30 V and 240 VA within 30 seconds of shutdown, protecting firefighters.

Blocking diode

A diode placed in series with a string or module to prevent reverse current from flowing into a lower-voltage or shaded string, protecting modules from overheating.

DC arc hazard

Unlike AC, DC arcs do not self-extinguish at a current zero crossing. Array conductors remain energized whenever illuminated, so DC circuits require rated disconnects and safe work practices.

Fall protection threshold

OSHA requires fall protection above 6 ft in general industry and 4 ft in construction. Roof anchors, harnesses, and lanyards are typical PV rooftop measures.

IV curve tracing

A commissioning and troubleshooting test that sweeps array voltage and records current. Comparing the traced curve to expected values reveals shading, mismatch, or failed modules.

Insulation resistance test

A megohmmeter test applying high DC voltage between conductors and ground to detect insulation degradation, moisture intrusion, or damaged cable before energizing the system.

Thermal imaging inspection

Infrared scanning of modules and terminations reveals hot spots, failed bypass diodes, and loose connections that show as temperature anomalies under load.

Equipment grounding

Bonding module frames, racking, and inverter enclosures to the grounding electrode system to clear faults and protect personnel, distinct from system conductor grounding.

String current comparison

Measuring each string's current under equal irradiance; a string reading about 10 % low suggests soiling, a bad connection, or a failed module substring.

Maintenance torque check

Periodically verifying termination torque on lugs, combiners, and module clamps prevents loose connections that cause resistance heating, voltage drop, and arc faults.