Solar Glossary

Electrical current that periodically reverses direction. The form of electricity delivered by the utility grid and used by household appliances. In the US, the standard is 120V/240V at 60 Hz.

One of two AEP Transmission & Distribution Utilities (TDUs) in ERCOT. AEP Central serves south and coastal Texas — Corpus Christi, Laredo, the Rio Grande Valley, and Victoria. Like every TDU it owns the poles and wires and charges regulated delivery fees on your bill regardless of which REP you pick.

The second AEP TDU in ERCOT, serving west-central Texas — Abilene, San Angelo, and surrounding areas. Same role as AEP Central: a regulated "wires" company whose delivery charges apply no matter your retail provider.

A protective device that detects the electrical signature of an arc fault — dangerous sparking caused by damaged or loose wiring — and shuts off power. Required by NEC 2017+ on certain PV system circuits to prevent fires.

A unit of battery capacity representing the charge stored when 1 amp flows for 1 hour. Multiply Ah by nominal voltage to get watt-hours (Wh). Example: a 100 Ah, 48V battery stores 4,800 Wh = 4.8 kWh.

The government body (city, county, or state) responsible for reviewing and approving solar permit applications and inspecting completed installations. The AHJ enforces the NEC and local building codes.

The fraction of solar radiation reflected by a surface. Snow has very high albedo (~0.8); asphalt is low (~0.1). Bifacial panels benefit from high-albedo ground surfaces behind and beneath the array.

A nonprofit organization that oversees the development of voluntary consensus standards for products, services, and systems in the US. Many electrical standards (e.g., ANSI C12 for meters) are developed under ANSI's framework.

A required safety function that causes a grid-tied inverter to shut down within milliseconds when it detects a grid outage. Prevents energizing utility lines while workers may be making repairs.

The complete set of solar panels installed at a site, including all strings and sub-arrays. An array is made up of individual modules wired together.

The US standard for wire conductor diameter. Counterintuitively, lower AWG numbers mean thicker wire: 10 AWG is thicker than 12 AWG. PV systems typically use 10–12 AWG for module wiring, 6–8 AWG for battery cables, and 2–4 AWG for main service connections.

The compass direction a solar panel faces, measured in degrees. 180° = due South (optimal for fixed panels in the Northern Hemisphere). East-facing panels (90°) produce more in the morning; West (270°) in the afternoon.

A panel design that generates power from both the front and rear surfaces. The rear side captures reflected (albedo) light from the ground or roof, typically adding 5–20% additional yield.

The electronic circuit board inside a battery pack that monitors cell voltages, temperature, and current. Protects against overcharge, over-discharge, short circuit, and thermal runaway. Communicates with the inverter via CAN bus or RS485.

All components of a solar installation other than the panels themselves — mounting hardware, wiring, conduit, inverter, disconnect switches, labor, and miscellaneous parts.

The rate at which a battery charges or discharges relative to its total capacity. 1C means full charge/discharge in 1 hour. 0.5C means 2 hours. LiFePO4 batteries are typically rated at 0.5C continuous, 1C peak.

A weighted average inverter efficiency that more accurately reflects real-world performance than peak efficiency. Weights seven operating points (10%–100% of rated power) to account for the time an inverter spends at partial load.

The smallest photovoltaic unit. Multiple cells are assembled into a module (panel). Most residential cells are monocrystalline silicon, typically 156–210 mm square.

The Transmission & Distribution Utility (TDU) serving the Houston area. A regulated monopoly that owns and maintains the power lines delivering electricity to your home, regardless of which REP you choose.

Occurs when the DC power from the array exceeds the inverter's maximum input capacity. The inverter "clips" (limits) output to its rated AC power, wasting some solar production. A small amount of clipping is often economically justified by undersizing the inverter (DC:AC ratio > 1.0).

A sensor that measures AC current by wrapping around a conductor and producing a proportional low-level signal. Used in solar monitoring systems to measure grid import/export and load consumption without interrupting the circuit.

The number of full charge/discharge cycles a battery can complete before capacity falls below a specified threshold (typically 80% of original). LiFePO4 batteries commonly achieve 3,000–6,000 cycles; lead-acid batteries typically 300–500 cycles.

Electrical current that flows in one direction. Solar panels produce DC; the inverter converts it to AC. Batteries store and release DC power.

The ratio of the array's DC nameplate capacity (kWp) to the inverter's rated AC output (kW). A ratio of 1.0–1.3 is typical. Higher ratios reduce cost per watt but increase clipping losses.

Sunlight that reaches a surface after being scattered by the atmosphere, clouds, or objects — arriving from all directions rather than directly from the sun. Panels still generate power from diffuse light, though at reduced output.

A safety switch that allows the system to be de-energized quickly by utility workers or first responders. NEC requires a readily accessible AC disconnect near the utility meter and a DC disconnect near the inverter.

The percentage of a battery's total capacity that has been discharged. Example: discharging 80% of a 10 kWh battery = 80% DoD, leaving 2 kWh remaining. Higher DoD increases cycle stress; most battery warranties specify maximum DoD.

Used by lenders to evaluate commercial solar loans. Ratio of net operating income to total debt service. A DSCR > 1.25 is typically required.

The independent system operator that manages the flow of electric power on the deregulated Texas grid, serving approximately 90% of the state's electric load. ERCOT publishes wholesale electricity prices by load zone in near-real time.

The unique 17–22 digit number ERCOT assigns to a specific electric meter / service address in deregulated Texas — effectively a fingerprint for your home's connection to the grid. You need it to switch Retail Electric Providers (REPs) and to pull your interval usage from Smart Meter Texas. The leading digits identify your TDU: CenterPoint = 1008901, Oncor = 1017699, AEP Central = 1003278, AEP North = 1020404, TNMP = 1040051, Lubbock P&L = 1011292. The remaining digits are unique to your meter.

Any system that stores energy for later use. In solar contexts, typically refers to a battery bank paired with an inverter/charger. An ESS can shift self-generated solar energy to evening hours when the panels aren't producing.

A European weighted average efficiency standard, similar to CEC efficiency. Weights six operating points (5%–100% of rated power), giving more emphasis to low-load operation. Common on European inverter spec sheets.

A program that pays solar owners a set rate for each kWh they export to the grid. Most of Texas uses REP-set buyback / net-billing instead of a formal feed-in tariff, but the term is common in solar worldwide.

A measure of solar cell quality: the ratio of a cell's actual maximum power (Vmpp × Impp) to its theoretical maximum (Voc × Isc). Higher is better — typical cells run 0.75–0.85. A low fill factor signals resistive losses or lower-quality cells.

A safety device that monitors current imbalance between hot and neutral conductors and trips within milliseconds if a ground fault is detected — indicating current is flowing through an unintended path (like a person). Required in bathrooms, kitchens, outdoors, and garages.

A ground-fault protection device specifically designed for PV arrays. Detects unintended current flow to ground in DC conductors and shuts down the array. Required by NEC 690.5 for grounded PV systems on buildings.

Total solar radiation received per unit area on a horizontal surface — the sum of direct beam radiation and diffuse sky radiation. Used as the primary input for solar resource assessment, measured in kWh/m².

An inverter/system that is connected to the utility grid and cannot operate independently during a power outage (unless it also has a battery and a transfer switch). The most common residential solar configuration.

An unintended electrical connection between a conductor and ground. In solar systems, ground faults can be caused by damaged wire insulation, moisture, or faulty equipment. Inverters include ground fault protection that shuts the system down if detected.

A solar array installed on racks secured to the ground rather than on a building rooftop. Allows optimal tilt/azimuth orientation and easier maintenance.

The total solar radiation incident on a tilted panel surface, accounting for the panel's tilt and orientation. GTI is what actually drives panel output; it is calculated from GHI using transposition models.

An inverter that manages both solar panels and a battery bank simultaneously. Handles charging, discharging, grid interaction, and load management in a single unit. Examples: EG4 6000XP, Sol-Ark 12K.

A factor that accounts for increased reflection losses when sunlight hits a panel at oblique angles (early morning, late afternoon, winter months). A panel reflecting 3% of light at normal incidence may reflect 15%+ at a 70° angle.

The US standard governing the interconnection of distributed energy resources (including solar) with the electric grid. Sets requirements for voltage/frequency ride-through, anti-islanding, reactive power control, and communication.

The DC current a module produces at the point of maximum power output under standard test conditions. Measured in amps (A).

The ratio of AC power output to DC power input, expressed as a percentage. Peak efficiency is measured at optimal load; weighted efficiencies (CEC, Euro) better represent real-world performance across varying conditions. Typical modern inverters: 96–99% peak efficiency.

An IEC standard (IEC 60529) that classifies the degree of protection an enclosure provides against solids and liquids. Written as "IP" followed by two digits: the first rates solid particle protection (0–6), the second rates liquid protection (0–9). Outdoor inverters and combiner boxes are typically rated IP65 (dust-tight, protected against water jets) or higher.

The annualized rate of return on a solar investment, accounting for all cash flows (upfront cost, annual savings, incentives) over the system's lifetime. A project with IRR > the cost of capital (e.g., mortgage rate) is financially favorable.

The maximum DC current a module produces when the output terminals are shorted (zero voltage). Used for wire sizing and fuse selection. Always greater than IMP.

A federal tax credit that reduces income tax liability dollar-for-dollar based on a percentage of the installed cost of a solar system. The 30% residential ITC (Section 25D) was eliminated in 2025; check current legislation for applicable rates. This is a tax credit, not a rebate — you need sufficient tax liability to use it.

The weatherproof enclosure on the back of a solar panel that houses the bypass diodes and connects the internal cell strings to the panel's output cables (the MC4 leads). A failing junction box is a common source of panel faults.

A unit of power equal to 1,000 watts — measuring the rate of energy flow at an instant in time. Not to be confused with kWh (energy). A 10 kW solar array produces 10 kW at peak, but over 5 hours of peak sun it produces 50 kWh of energy. Inverters are rated in kW (power capacity); batteries are rated in kWh (energy capacity).

The standard unit of electrical energy. 1 kWh = 1 kilowatt of power delivered or consumed for 1 hour. Used for battery capacity, utility billing (cost per kWh), and annual solar production.

The nameplate DC power output of a solar module or array under standard test conditions (1,000 W/m² irradiance, 25°C cell temperature). The standard unit for sizing a solar system.

The lifetime cost of a solar system divided by the total energy it produces, expressed in $/kWh. Provides an apples-to-apples comparison with utility electricity rates. Typical residential solar LCOE is $0.06–0.10/kWh.

The oldest rechargeable battery chemistry. Flooded, AGM, and gel variants exist. Cheaper upfront but heavier, shorter cycle life (300–500 cycles), limited DoD (50%), and higher maintenance than lithium. Being rapidly displaced by LiFePO4 in solar applications.

A small, permanent power loss (typically 1–3%) that occurs in silicon cells during the first hours of sunlight exposure due to boron-oxygen defects in the crystal structure.

The dominant lithium battery chemistry for solar storage. Safer than other lithium chemistries (does not catch fire if punctured), long cycle life (3,000–6,000+ cycles), wide temperature range, and flat discharge curve. Used in EG4, Signature Solar, Fortress, and most residential solar batteries.

ERCOT divides Texas into four settlement point load zones: LZ_HOUSTON (Houston area), LZ_NORTH (DFW / North Texas), LZ_SOUTH (South Texas / Corpus Christi), and LZ_WEST (West Texas). Wholesale electricity prices differ by zone based on local generation mix and transmission constraints.

The municipal utility serving Lubbock. Most of its load has been integrated into the ERCOT competitive market, so Lubbock customers can now choose a Retail Electric Provider (REP) while LP&L acts as the local TDU — the wires company that sets regulated delivery charges.

A US tax depreciation method that allows commercial solar assets to be depreciated over 5 years. Combined with the ITC, MACRS can dramatically improve commercial solar economics.

A small inverter mounted on each individual panel (rather than one central inverter). Converts DC to AC at the module level. Advantage: each panel operates independently so shading or mismatch on one doesn't drag the others down, and every panel is monitored individually. Highest per-watt cost; great for shaded or complex roofs. Example: Enphase IQ series (also APsystems, Hoymiles).

A transfer switch device that manages the boundary between a home's critical loads, the solar/battery system, and the utility grid. Allows islanding (operating off-grid during an outage) while meeting IEEE 1547 anti-islanding requirements.

Power lost when modules in a string have slightly different electrical characteristics (due to manufacturing tolerances, partial shading, soiling, or aging). The weakest module limits current for the whole string. Typically 1–3% of system output.

Umbrella term for electronics that manage each panel individually rather than a whole string — namely power optimizers and micro-inverters. MLPE raises output on shaded or multi-orientation roofs (one weak panel no longer limits the rest) and adds per-panel monitoring, at higher cost than a plain string inverter. Rule of thumb: clean, unshaded, single-orientation roof → string inverter; shade or several orientations → optimizers or micros.

A single solar panel — the finished, weather-sealed unit consisting of multiple cells laminated between glass and a backsheet. Also called a "panel."

The dominant cell technology for residential solar. Made from a single silicon crystal, giving higher efficiency (19–23%) and a uniform black appearance. More expensive than polycrystalline but produces more power per square foot.

An algorithm built into inverters and charge controllers that continuously adjusts the operating voltage of a solar string to extract the maximum available power, accounting for changing irradiance, temperature, and shading conditions.

The widely adopted US standard for electrical safety in buildings, published by NFPA. Article 690 of the NEC specifically covers photovoltaic systems. Most jurisdictions adopt the NEC, sometimes with local amendments.

The US trade association that publishes standards for electrical enclosures and equipment. NEMA enclosure ratings indicate environmental protection: NEMA 3R (rain-proof, for outdoor use), NEMA 4 (watertight), NEMA 4X (watertight + corrosion-resistant). Solar combiner boxes and disconnect switches are often NEMA 3R or 4X.

A utility billing arrangement where excess solar production exported to the grid is credited against future electricity bills at (or near) the retail rate. Texas does not require net metering statewide — credit rates vary by REP.

The nonprofit organization that publishes the National Electrical Code (NEC) and NFPA 70E (electrical safety in the workplace). NFPA 855 specifically covers installation of stationary energy storage systems (batteries).

The expected cell temperature under moderate, real-world conditions: 800 W/m² irradiance, 20°C ambient temperature, and 1 m/s wind speed. Used to estimate actual output versus STC ratings.

The present value of all future cash flows from a solar investment, minus the initial cost. A positive NPV means the investment creates value; negative NPV means it destroys value at the assumed discount rate.

A solar system with no connection to the utility grid. Must be sized with enough battery storage and/or generator backup to meet all loads independently. Common for rural properties without grid access.

The TDU serving the Dallas-Fort Worth metroplex and much of North Texas. Like CenterPoint, Oncor maintains the "wires" — the actual delivery infrastructure — while REPs handle retail energy sales.

Probabilistic production estimates. P50 = expected production in an average year (50% of years will produce more, 50% less). P90 = production level exceeded in 90% of years — a conservative "downside" estimate used by lenders.

A feature of hybrid inverters that provides backup power to a critical loads panel during a grid outage, drawing from battery storage. Response time is typically < 20 ms.

The rated peak power of a module under standard test conditions, in watts (W). The number on the spec sheet label — e.g., a "400W panel" has Pnom = 400 W.

An older cell technology made from multiple silicon crystals melted together. Slightly lower efficiency (15–18%) than monocrystalline; identifiable by a blue, speckled appearance. Largely replaced by monocrystalline in modern installs.

The ratio of real power (W) to apparent power (VA) in an AC circuit. A PF of 1.0 means all power delivered does useful work. Inverters are typically rated at unity (1.0) power factor. Low PF from inductive loads (motors, compressors) can cause utility penalties for commercial customers.

A small module-level device attached behind each panel that does per-panel maximum power point tracking (MPPT). Panels still connect in a string to a central inverter, but each optimizer decouples its panel electrically — so shade or a weak panel is throttled on its own instead of dragging the whole string down, and you get per-panel monitoring. Common with SolarEdge (integrated) and Tigo (retrofit onto a standard string inverter).

A financing arrangement where a third party owns and maintains the solar system on your property and sells you the power at a contracted rate, typically below retail. No upfront cost, but you don't own the system or receive tax credits.

A key metric for solar system quality: actual energy output divided by the theoretical output if the system operated at STC efficiency for all available irradiance. A well-designed, well-installed residential system achieves PR of 75–85%. Lower PR indicates losses from temperature, soiling, shading, wiring, or inverter inefficiency.

The state agency that regulates electric and telecommunications utilities in Texas. Oversees REP licensing, retail competition rules, and TDU rates. Also oversees grid reliability coordination with ERCOT.

The technology that converts light directly into electricity using semiconductor materials. The "photo" refers to light; "voltaic" refers to voltage/electricity. The photovoltaic effect was discovered by Edmond Becquerel in 1839. PV is distinct from solar thermal, which uses sunlight to heat water or air.

The fraction of photons striking a solar cell (at a given wavelength) that get converted into collected charge carriers. It describes how effectively a cell turns each color of light into current across the spectrum — a core measure of cell performance.

An NEC requirement (705.12) that rooftop solar systems be able to reduce conductor voltage in the array to a safe level (< 80V) within 30 seconds of initiating shutdown. Required for firefighter safety on buildings.

A licensed company that sells electricity to end customers in deregulated Texas markets. REPs buy wholesale power from ERCOT and compete on price and plan features. Examples: Reliant, TXU Energy, Rhythm, Pulse Power. Your solar buyback credit rate is set by your REP, not the TDU.

The percentage of energy you can get out of a battery relative to what you put in, accounting for charge and discharge losses. Typical LiFePO4 round-trip efficiency is 95–98%; lead-acid is 70–85%.

Texas Comptroller rule exempting solar energy equipment from state sales tax when the system is used to generate electricity. This means panels, inverters, batteries, racking, and wiring for a solar install are not subject to Texas' 8.25% sales tax.

The fraction of solar energy produced that is consumed on-site rather than exported to the grid. Higher self-consumption is financially beneficial when buyback rates are lower than retail rates — which is the case for most Texas REP plans.

The fraction of a building's total electricity demand that is met by solar production (directly or via battery storage). A 100% self-sufficient system would eliminate the electricity bill entirely.

A type of Texas electricity plan where your REP credits exported solar power at or near the wholesale ERCOT spot price rather than a fixed buyback rate. Value varies hour by hour based on grid conditions.

The number of years it takes for cumulative energy savings to equal the upfront system cost. Calculated as: total cost ÷ annual savings. Does not account for rate escalation or the time value of money — use NPV for a more rigorous analysis.

A simplified electrical schematic that shows the main components of a solar system (panels, inverter, disconnect, meter, panel) and how they connect, using single lines to represent conductors. Required for permits.

A state-run portal (smartmetertexas.com) where Texas electricity customers can download their 15-minute interval usage data from their smart meter. This data can be uploaded to solar design tools to accurately model energy use.

The current charge level of a battery expressed as a percentage of full capacity. 100% = fully charged, 0% = fully discharged (though most systems limit discharge to a minimum SoC to protect the battery).

Power reduction caused by dirt, dust, pollen, bird droppings, or other debris accumulating on panel surfaces. Typically 1–5% in most US climates; can exceed 25% in dry, dusty environments without regular cleaning.

Annual energy production per unit of installed capacity, expressed in kWh/kWp per year. A useful metric for comparing sites — Houston averages roughly 1,400–1,550 kWh/kWp; Phoenix averages 1,700–1,900 kWh/kWp.

The standard US residential electrical service: two 120V legs that combine to provide 240V for large appliances (A/C, dryer, water heater). Inverters designed for US homes must output split-phase 120/240V.

ERCOT's wholesale electricity price at a given location and time, published in $/MWh. The price varies by load zone (LZ_HOUSTON, LZ_NORTH, etc.) and changes every 15 minutes based on supply and demand.

The laboratory conditions under which solar module output is rated: 1,000 W/m² irradiance, 25°C cell temperature, and AM 1.5 spectrum. Real-world output is almost always lower.

A series of solar modules wired positive-to-negative to increase voltage. Voltage adds across a string while current stays the same. A string feeds into one MPPT input of the inverter.

A central inverter that accepts DC power from one or more strings of series-wired modules. The most common and cost-effective topology for systems without significant shading.

The regulated "wires company" that physically delivers electricity to your home through the poles and wires infrastructure. In Texas, your TDU is determined by your location — you cannot choose. TDU charges appear as fixed monthly and per-kWh delivery fees on your bill regardless of your REP.

The rate at which a module's power output changes with temperature, expressed as % per °C (µPmpp). Most silicon panels lose about −0.35% to −0.45% of rated power for each °C above 25°C. In Texas summer heat (cell temps of 60–70°C), this can represent a 12–20% output reduction.

A measure of how cleanly an inverter produces a sine wave. Lower THD means cleaner AC output. Grid-tied inverters typically achieve < 3% THD; poor inverters may have > 5% THD, which can cause issues with sensitive electronics.

The angle of a solar panel relative to horizontal, measured in degrees. A 0° tilt is flat; 90° is vertical. The optimal fixed tilt for maximum annual production roughly equals the site's latitude. In Texas (29–35° N), optimal tilt is typically 25–32°.

A Transmission & Distribution Utility (TDU) in ERCOT serving scattered parts of Texas — pockets of the Gulf Coast near Houston (e.g., Texas City, League City), North Central Texas, and West Texas. Owns the local delivery infrastructure and sets regulated TDU charges.

An independent safety certification organization. Solar inverters and modules are typically UL-listed (UL 1741 for inverters, UL 61730 for modules) to demonstrate they meet safety standards required for interconnection approval.

A device that provides near-instant backup power from a battery when grid power fails, protecting sensitive equipment from outages and surges. Traditional UPS units are small and short-duration (minutes). A solar+battery system with EPS/transfer switch functionality is essentially a whole-home UPS with hours of runtime.

The DC voltage at which a module (or string) produces its maximum power under standard test conditions. Used to calculate how many modules to put in series for a given inverter MPPT range.

The maximum voltage a module produces when no current is flowing (nothing connected to the terminals). Used for string sizing — the string VOC at minimum temperature must not exceed the inverter's maximum input voltage.

Power lost as heat in DC and AC conductors due to electrical resistance. Proper wire sizing keeps wiring losses below 1–2%. Also called ohmic loss or resistive loss.

A tough, heat- and UV-resistant insulation used on DC "PV wire" and other solar conductors. Its durability under sunlight and high temperatures makes it a standard choice for wiring between panels and to the inverter.

A system's energy output. The most-used form is "specific yield" — annual energy per unit of installed capacity (kWh per kWp per year) — which normalizes output by system size so designs and locations compare fairly. Texas systems typically see ~1,400–1,700 kWh/kWp.

The angle between the sun and straight overhead (the vertical) — equal to 90° minus the sun's elevation. A high sun has a small zenith angle; a low sun a large one. Larger zenith angles mean sunlight passes through more atmosphere, reducing its intensity on your panels.