NABCEP SW Associate cheat sheet

Standard Test Conditions (STC): 1000 W/m² irradiance, 25 °C cell temperature, AM 1.5 spectrum; nameplate ratings use STC, real output is usually lower.

NOCT (Nominal Operating Cell Temperature) is measured at 800 W/m², 20 °C ambient, 1 m/s wind; typical values 42-48 °C for framed modules.

Cell temperature estimate: Tcell ≈ Tambient + (NOCT − 20) × G/800, with G in W/m².

Power temperature coefficient for crystalline silicon is about −0.3 to −0.45 %/°C, so a module at 65 °C loses roughly 12-18 % of STC power.

Irradiance and current are nearly proportional; cell temperature mainly shifts voltage, lowering Vmp as temperature rises.

Peak sun hours (PSH) equal daily kWh/m² of irradiation; a 5 kW array in 5 PSH area produces about 25 kWh/day before derating.

Performance ratio (PR) = actual AC energy / (array STC kW × plane-of-array irradiation in kWh/m²); good PR is 0.75-0.85.

Orientation rule for fixed tilt in the northern hemisphere: face true south; tilt near latitude, steeper tilt favors winter yield, shallower favors summer.

Shading of even one cell substring can collapse string current; bypass diodes limit loss to a substring, blocking diodes prevent reverse current.

Ohm's law: V = I × R; power P = V × I = I²R = V²/R.

Series strings add voltages (Vtotal = ΣV) while current stays the same; parallel connections add currents while voltage stays the same.

Power in a series string is limited by the lowest-current module; in parallel, by the lowest-voltage source.

Energy unit conversions: 1 kWh = 1000 Wh; 1 kWh = 3.6 MJ; 1 hp ≈ 0.746 kW.

Ampacity: continuous PV currents must be multiplied by 1.25 for conductor and overcurrent sizing (NEC 690.8).

Voltage drop for DC circuits: Vdrop = 2 × L × I × Rconductor (round trip); keep DC drop under about 2-3 % for design quality.

Standard array voltages: 12 V, 24 V, 48 V nominal battery banks; grid-tied string voltages commonly 300-600 V DC.

Insolation is energy per area per day (kWh/m²/day); irradiance is instantaneous power per area (W/m²).

AC single-phase power: P = V × I × power factor; three-phase: P = √3 × V × I × power factor.

Module I-V curve: Isc at V=0, Voc at I=0, and the maximum power point where Imp × Vmp is largest; fill factor = Pmax/(Voc × Isc).

String sizing: cold-temperature Voc (with temperature coefficient) must stay below inverter maximum DC input; hot-temperature Vmp must stay above inverter MPPT minimum.

Inverter DC/AC ratio (oversizing) of 1.1-1.3 is common; clipping occurs when array DC power exceeds inverter AC rating.

Battery capacity: usable kWh = nominal Ah × nominal V × depth of discharge; lead-acid DoD typically 50 %, lithium 80-100 %.

Charge controllers: PWM is simple and forces array to battery voltage; MPPT tracks the array maximum power point and allows higher array voltage.

Bypass diodes are inside modules/junction boxes; blocking diodes sit in the string or combiner to stop reverse current into a shaded string.

Rapid shutdown per NEC 690.12 requires conductors within 1 ft of the array and 30 V/240 VA limit within 30 seconds of shutdown.

Ground-fault protection (NEC 690.5) is required for grounded DC systems; arc-fault circuit interruption (690.11) is required on DC circuits above 80 V.

Conductor ampacity for PV source and output circuits: Isc × 1.25, then apply temperature and conduit fill correction factors.

Fall protection is required at 6 ft (OSHA general industry) or 4 ft (construction) above a lower level; roof anchors and harnesses are standard.

DC arc hazard persists whenever the array is illuminated; treat all DC conductors as live and use a DC-rated meter and insulated tools.

Module open-circuit voltage rises in cold weather; measure Voc and compare to expected temperature-corrected value to spot failed strings.

IV curve tracing and insulation resistance (megohmmeter) testing are standard commissioning and troubleshooting methods.

Thermal imaging finds hot spots, bypass-diode failures, and loose terminations; a hot cell under reverse bias can reach damaging temperatures.

Grounding: equipment grounding conductor bonds module frames, racking, and inverter enclosure; system grounding depends on inverter type (transformerless = ungrounded).

Roof clearance and setback: NEC 690.13/705.12 and fire code require labeled disconnects and reserved access pathways on residential roofs.

Soiling, snow, and shading losses are checked by comparing string current under the same irradiance; a string 10 % low suggests soiling or a bad connection.

Periodic maintenance: torque terminations, inspect for rodent damage and water intrusion, verify inverter fault logs, and check surge protection status.