Solar Engineering Glossary

AC (Alternating Current)

The type of electrical current that alternates direction 60 times per second (60Hz) in the U.S. Solar inverters convert DC power from panels to AC power compatible with the grid and home appliances.

REF: NEC Article 690 / IEEE Standard

AFCI (Arc-Fault Circuit Interrupter)

A device required by NEC 690.11 for DC circuits above 80V. AFCIs detect dangerous arc faults — unintended electrical arcing in wiring — and interrupt the circuit to prevent fires. Required on all DC circuits for systems over 80V.

REF: NEC 690.11 (2017+)

AHJ (Authority Having Jurisdiction)

The local government agency responsible for enforcing building and electrical codes — typically the city or county building department. The AHJ reviews permit plan sets, conducts inspections, and issues final certificates of occupancy.

REF: NEC 90.2 / IBC 105

ASCE 7-16 (American Society of Civil Engineers, Minimum Design Loads)

The primary structural engineering standard for calculating wind, snow, seismic, and dead loads on buildings and solar racking systems. The "7-16" refers to the 2016 edition. Wind velocity pressure formula: q = 0.00256 × Kz × Kzt × Kd × V².

REF: ASCE 7-16 Chapters 26-30

Backfeed Breaker

The circuit breaker in the main electrical panel through which solar power flows from the inverter to the panel bus. Its maximum size is limited by the NEC 705.12 120% Busbar Rule: (Bus Rating × 1.20) − Main Breaker Size.

REF: NEC 705.12(B)(2)

Battery Energy Storage System (BESS / ESS)

A system of battery cells that stores electrical energy for later use. Solar + storage systems use batteries to store daytime solar generation for use at night or during outages. Governed by NEC Article 706 and NFPA 855 for installation safety.

REF: NEC 706 / NFPA 855

Bus Rating (Busbar Rating)

The maximum current rating of the copper or aluminum bus bar inside a main electrical panel. Critical for the NEC 705.12 120% Busbar Rule calculation. Common residential ratings: 100A, 125A, 150A, 200A, 225A. Never confused with the main breaker size.

REF: NEC 705.12(B)

CalFire / IFC Roof Access Requirements

California and IFC Section 605.11 require a minimum 3-foot-wide access pathway from the eave to the ridge, and along the ridge, for firefighter access. Solar panels must maintain 18" setbacks from roof edges and valleys in most jurisdictions.

REF: IFC §605.11 / CFC §605.11

CEC (California Energy Commission)

The state agency that administers California's Building Energy Efficiency Standards (Title 24 Part 6). The CEC mandates solar PV on all new low-rise residential buildings (since 2020) and battery storage systems on new single-family homes built under the 2022 Title 24 code.

REF: Title 24 Part 6 / CEC

Conduit Fill Derating

When multiple current-carrying conductors share a single conduit, each conductor's ampacity is reduced. NEC Table 310.15(C)(1) specifies the derating factor: 4-6 conductors = 80%, 7-9 conductors = 70%, 10-20 conductors = 50%.

REF: NEC Table 310.15(C)(1)

DC (Direct Current)

Electrical current that flows in one direction only, produced by solar photovoltaic panels. Solar systems produce DC power which is then converted to AC by the inverter for use in buildings and the grid.

REF: NEC Article 690

DC-to-AC Ratio (DC:AC Ratio)

The ratio of total DC array capacity (kWp) to inverter AC output capacity (kWac). A ratio of 1.25 means a 10kW inverter is paired with a 12.5kW array. Industry standard range is 1.15–1.35 for residential, up to 1.50 for commercial systems.

REF: IEC 61724 / NREL PVWatts

Dead Load

The permanent static weight of all building components and attached equipment. For solar installations, dead load includes the weight of panels, racking, and conduit added to the existing roof structure. Typically 2–4 pounds per square foot (psf) for residential solar arrays.

REF: ASCE 7-16 §3 / IBC 1606

Exposure Category (ASCE 7-16)

A classification of the site terrain that affects wind load calculations. Category B = urban/suburban with obstructions. Category C = open terrain with few obstructions. Category D = flat, unobstructed areas near water. Most residential installations are Exposure B or C.

REF: ASCE 7-16 §26.7

GEC (Grounding Electrode Conductor)

The conductor connecting the inverter's grounding system to the grounding electrode (ground rod or ground ring). Required to safely dissipate fault currents and lightning surges to earth. Sized per NEC 250.64.

REF: NEC 250.64 / NEC 690.47

Ground-Mount Solar Array

A solar PV system installed on a freestanding structural racking system anchored to the ground via concrete piers, helical piles, or ballast. Requires geotechnical (soil) analysis, concrete pier design, and utility easement review as part of the permit plan set.

REF: IBC 1807 / ACI 318

Imp (Current at Maximum Power)

The current produced by a solar module at its maximum power point (MPP) under Standard Test Conditions (STC: 1000 W/m², 25°C). Used to calculate operating string current. Found on the module's specification sheet.

Isc (Short Circuit Current)

The maximum current a solar module can produce when its output terminals are short-circuited. Used in NEC 690.8 conductor sizing: I_continuous = Isc × strings × 1.5625. The Isc value is found on the module's STC specification label.

REF: NEC 690.8(A)

Interconnection Agreement

A formal contract between the solar system owner and the utility company that permits the solar system to connect to the electrical grid. Required before energization. Application is submitted separately from the building permit and typically processed by the utility's interconnection team.

REF: FERC Order 2003 / State PUC

Inverter

The core power electronics component that converts DC electricity from solar panels to AC electricity. Types include: String Inverters (1 per string), Microinverters (1 per panel), Power Optimizers + String Inverter, and Central Inverters (utility-scale). Inverter specifications determine maximum system voltage and AC output capacity.

REF: UL 1741 / IEEE 1547

Lag Bolt Withdrawal Capacity

The force required to pull a lag bolt straight out of the wood framing member (rafter). Calculated per AWC NDS Table 11.2: W' = Cd × CM × Ct × W × l_e, where l_e is effective embedment depth and W is reference withdrawal design value. Embedment must be ≥ 2.5 inches.

REF: AWC NDS §11.2 / IBC

MPP (Maximum Power Point)

The operating point on a solar module's I-V curve where the product of voltage (Vmp) and current (Imp) is maximized, producing maximum power output. MPPT inverters continuously track and operate at the MPP to maximize energy harvest.

MPU (Main Panel Upgrade)

The replacement or upgrade of an existing electrical service panel, typically triggered when the existing panel's busbar capacity is insufficient to accommodate the solar backfeed breaker under the NEC 705.12 120% Busbar Rule. A common upgrade is from a 100A/150A panel to a 200A panel.

REF: NEC 705.12 / NEC 230

NEC (National Electrical Code)

The NFPA 70 standard — adopted by most U.S. states — governing electrical wiring and equipment. Article 690 covers Solar Photovoltaic (PV) Systems. Article 705 covers Interconnected Electric Power Production Sources. Updated every 3 years; current edition is NEC 2023, with many states on 2017 or 2020.

REF: NFPA 70 — NEC

NEM (Net Energy Metering)

A utility billing mechanism that credits solar customers for excess electricity exported to the grid. NEM 2.0 (older CA program) credits at retail rate. NEM 3.0 (Net Billing Tariff, effective 2023 in CA) credits at a much lower avoided cost rate, making battery storage economically essential.

REF: CPUC Decision 22-12-056

NFPA 855 (Standard for Energy Storage Systems)

The National Fire Protection Association standard governing the installation of battery energy storage systems. Key limits: 80 kWh per control area for lithium-ion storage in residential, and specific separation distances and ventilation requirements for larger installations.

REF: NFPA 855 §15

P.E. Stamp (Professional Engineer Stamp)

The official seal and signature of a licensed Professional Engineer (P.E.) applied to engineering drawings, calculations, or documents. Required by many AHJs for solar permit plan sets. Wet stamps (physical ink) and digital/electronic stamps are accepted depending on jurisdiction.

REF: State Engineering Practice Acts

POI (Point of Interconnection)

The electrical point where a solar PV system connects to the utility grid or building electrical system. Must be clearly labeled on the Single Line Diagram (SLD) per NEC 705.12. For residential, this is typically the main service panel's backfeed breaker location.

REF: NEC 705.12 / IEEE 1547

Pmax (Maximum Power)

The maximum rated power output of a solar module measured in Watts (W) under Standard Test Conditions (STC). Also called Wp (Watts peak). Used to calculate total array DC capacity: Total DC = Pmax × Number of Modules.

psf (Pounds per Square Foot)

The unit of pressure or load used in structural engineering. Solar panels typically weigh 2–4 psf. Ground snow loads (Pg) range from 0 psf (Southern California) to 70+ psf in mountain areas. Wind pressures are also expressed in psf.

REF: ASCE 7-16

Rapid Shutdown (NEC 690.12)

A NEC 690.12 requirement that solar systems can be de-energized quickly for firefighter safety. Systems on buildings must reduce conductor voltage to 30V or less within 30 seconds of initiating shutdown. Requires a rapid shutdown initiator device at or near the utility meter or main service panel.

REF: NEC 690.12 (2017/2020)

Rafter

The diagonal sloped structural member of a roof that supports the roof sheathing, roofing material, and solar panel loads. Rafter size, species, and spacing are documented in the structural plan set to verify adequacy under additional solar dead load and wind uplift forces.

REF: AWC Span Tables / IBC 2308

SLD (Single Line Diagram)

An electrical drawing that uses single lines to represent multi-conductor circuits and symbols to represent electrical components. Required for all solar permit plan sets. Must show panels, inverter, disconnects, conductors with gauge/conduit type, overcurrent protection, grounding, and the POI — all per NEC 690.

REF: NEC 690 / AHJ Plan Check

STC (Standard Test Conditions)

The laboratory conditions under which solar module performance is rated: 1,000 W/m² irradiance, 25°C cell temperature, and AM 1.5 spectrum. Real-world performance typically differs from STC due to ambient temperatures, shading, and irradiance variation.

REF: IEC 61215 / IEC 61730

String (Solar String)

A series-connected group of solar modules. Modules connected in series add their voltages while current remains the same. String voltage = Voc × number of modules. Maximum string voltage must not exceed inverter's rated input voltage or NEC 690.7 limits.

REF: NEC 690.7

Title 24 Part 6 (California Building Energy Efficiency Standards)

California's energy efficiency code for new construction and major renovations. Since 2020, it mandates solar PV on all new low-rise residential buildings. The 2022 edition (effective 2023) additionally mandates battery storage on new single-family homes, calculated based on conditioned floor area.

REF: CEC Title 24 Part 6 — 2022

USE-2 / PV Wire

Sunlight-resistant, moisture-resistant cable types rated for direct burial and outdoor solar DC wiring. USE-2 is rated 90°C and UL listed for solar PV applications. PV Wire is specifically designed for rooftop solar and is rated for higher voltages (600V or 1000V). Required for all rooftop DC wiring.

REF: NEC 690.31 / UL 4703

Vmp (Voltage at Maximum Power)

The voltage at which a solar module produces its maximum power output under STC conditions. Used to verify string voltage falls within the inverter's MPPT operating range. String Vmp = Vmp × modules per string. Must remain within inverter's MPPT window across all temperature conditions.

Voc (Open Circuit Voltage)

The maximum voltage a solar module produces when no current is flowing (open circuit). Used to calculate maximum system voltage per NEC 690.7. Temperature correction is critical: at low temperatures, Voc increases. Formula: V_max = Voc × modules × [1 + (Tc × (T_min − 25°C) / 100)].

REF: NEC 690.7

Wind Uplift Pressure

The upward force that wind exerts on a roof or solar array, calculated per ASCE 7-16. For solar panels, wind uplift pressure is calculated at field zones (Zone 1), edge zones (Zone 2), and corner zones (Zone 3) of the roof. Corner zones typically experience 2–3× higher uplift than field zones.

REF: ASCE 7-16 Chapter 30

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