Solar Acronyms Glossary 2026: GHI, DNI, DHI, POA, PR, LCOE, IRR & 50+ More Defined

Walk onto any solar construction site, sit in any design review, or read any utility-scale tender, and you will hear 70 acronyms in 90 minutes. GHI, DNI, DHI, POA, PR, CUF, LCOE, IRR, NPV, STC, NOCT, MPPT, BESS, AHJ, NEM, PTO, EPC, BoS, ITC, AFCI, RSD. Solar borrows from electrical engineering, atmospheric science, finance, and building code — and each domain brings its own vocabulary.

This guide defines the 70+ acronyms a working solar professional uses in 2026, organized by category. Every entry includes the expansion, a one-sentence definition, a practical example with real numbers, and (where relevant) the formula. Bookmark this page. Send it to your new hires. Use the master lookup table at the end when you need to decode a tender document fast.

In this guide:

• The 12 acronym categories every solar engineer needs to know
• Irradiance and resource: GHI, DNI, DHI, POA, TMY, AM, AOI, ETR
• Module and cell performance: STC, NOCT, PTC, Voc, Isc, Vmp, Imp, Pmax
• Electrical: AC, DC, kW, kWh, kWp, V, A, MPPT, OCPD, VAr
• Inverter and MLPE: MPPT, MLPE, AFCI, GFCI, RSD, GTI
• System performance: PR, CUF, P50, P90, P95, yield, capacity factor
• Finance: LCOE, IRR, NPV, ROI, PPA, ITC, MACRS, SREC, REC, RPS, FIT
• Project and construction: EPC, O&M, BoS, PV, BIPV, BAPV, BOM, COD, PTO, NTP
• Grid and interconnection: NEM, BTM, FTM, VPP, DER, DERMS, IPP, IEEE 1547
• Battery and storage: BESS, DoD, SoC, RTE, LFP, NMC, C-rate, EOL
• Compliance and code: NEC, UL, IEC, OSHA, NABCEP, AHJ, MCS
• Software and design: CAD, BIM, GIS, CFD, SCADA, PVsyst
• Master alphabetical lookup table — all 70+ acronyms in one place

Why the Solar Industry Has So Many Acronyms

Solar sits at the intersection of four mature technical fields. Electrical engineering brings AC, DC, MPPT, OCPD, and VAr. Atmospheric science brings GHI, DNI, DHI, AM, and AOI. Finance brings LCOE, IRR, NPV, and PPA. Building code brings NEC, NEC 690, UL, AHJ, and PTO. Stack those four vocabularies onto one product — a grid-connected PV plant — and you get 200+ acronyms in active use.

The cost of misreading one acronym is real. Confusing PR (Performance Ratio) with CUF (Capacity Utilization Factor) can swing a financial model by 30%. Confusing kW (instantaneous power) with kWh (energy over time) is the single most common error in residential proposals. Confusing STC (Standard Test Conditions) with NOCT (Nominal Operating Cell Temperature) overstates real output by 12–18%.

This guide is the reference I give to every new engineer I hire. Read it once, cross-check it against datasheets and tender documents, and the vocabulary stops blocking the work.

Irradiance and Solar Resource Acronyms

Solar irradiance is the energy flux from the sun hitting a surface, measured in W/m² (watts per square meter) or kWh/m² (kilowatt-hours per square meter) over a year. Five acronyms — GHI, DNI, DHI, POA, and TMY — drive every yield model.

GHI — Global Horizontal Irradiance

Expansion: Global Horizontal Irradiance Units: W/m² (instantaneous) or kWh/m²/day or kWh/m²/year Definition: Total solar irradiance on a flat horizontal surface, summing the direct beam and diffuse sky components.

GHI is the most widely reported solar resource value. Phoenix, Arizona averages 5.7 kWh/m²/day GHI. Berlin, Germany averages 2.8 kWh/m²/day. Lagos, Nigeria averages 5.0 kWh/m²/day. For deeper detail see our dedicated guide on GHI, DNI, and DHI in solar irradiance modeling and the GHI glossary entry.

Formula:

GHI = DNI × cos(SZA) + DHI

Where SZA is solar zenith angle.

DNI — Direct Normal Irradiance

Expansion: Direct Normal Irradiance Units: W/m² or kWh/m²/year Definition: Beam irradiance measured on a surface perpendicular to the sun’s rays, excluding any diffuse component.

DNI is the resource value that drives concentrating solar power (CSP) and high-DNI tracker projects. Atacama Desert, Chile measures 3,200+ kWh/m²/year DNI — the highest on Earth. Northern Europe DNI sits at 800–1,200 kWh/m²/year. See the DNI glossary entry for full detail.

DHI — Diffuse Horizontal Irradiance

Expansion: Diffuse Horizontal Irradiance Units: W/m² or kWh/m²/year Definition: Sky-scattered solar irradiance on a horizontal surface, excluding the direct beam component.

DHI dominates in cloudy climates. London averages 55% diffuse fraction (DHI/GHI). Phoenix averages 22%. DHI matters for bifacial gain modeling and for accurate output prediction in northern latitudes. Detail in the DHI glossary entry.

POA — Plane of Array Irradiance

Expansion: Plane of Array Irradiance Units: W/m² or kWh/m²/year Definition: Total solar irradiance hitting the actual tilted module surface, computed from GHI, DNI, and DHI using array geometry.

POA is what panels actually convert to electricity. A south-facing 30° tilt in Atlanta receives 1,650 kWh/m²/year POA versus 1,520 kWh/m²/year GHI — the 8.5% uplift comes from optimizing tilt for the sun’s average elevation. See the POA irradiance glossary entry and the plane of array irradiance glossary.

Formula (simplified):

POA = POA_beam + POA_diffuse + POA_reflected

Each component is computed using transposition models (Perez, Hay-Davies, Isotropic).

TMY — Typical Meteorological Year

Expansion: Typical Meteorological Year Definition: A synthetic year of hourly weather data assembled from 10–30 years of measurements, selected to represent typical long-term climate.

TMY3 is the most common US format. PVGIS-TMY covers Europe and Africa. SolarGIS and Meteonorm produce commercial TMY datasets. NREL’s National Solar Radiation Database (NSRDB) provides free TMY data for all US locations.

AM — Air Mass

Expansion: Air Mass Definition: A measure of the path length sunlight travels through Earth’s atmosphere, relative to the path at zenith.

AM 1.0 means the sun is directly overhead. AM 1.5 (zenith angle 48.2°) is the global standard for solar panel rating because it represents typical mid-latitude conditions. AM 0 is outside the atmosphere — 1,361 W/m².

AOI — Angle of Incidence

Expansion: Angle of Incidence Definition: The angle between the incoming sunlight and the surface normal of the solar panel.

AOI of 0° means light hits the panel perpendicular — maximum transmission. AOI above 60° causes reflection losses (IAM — Incidence Angle Modifier — quantifies these losses). See the angle of incidence glossary entry.

ETR — Extraterrestrial Radiation

Expansion: Extraterrestrial Radiation Definition: Solar irradiance at the top of Earth’s atmosphere, averaging 1,361 W/m² (the solar constant) with ±3.4% annual variation due to Earth’s elliptical orbit.

ETR sets the theoretical ceiling for all surface-level irradiance values.

NSRDB — National Solar Radiation Database

Expansion: National Solar Radiation Database Definition: NREL’s free, publicly accessible US irradiance database covering 1998–present at 4 km × 4 km × 30-minute resolution.

NSRDB feeds most US commercial solar yield models including SAM (System Advisor Model), PVsyst with US weather, and SurgePV’s US energy modeling. For accurate site-level resource assessment, solar shadow analysis software combines NSRDB irradiance with site-specific shading.

Module and Cell Performance Acronyms

Every solar panel datasheet uses 12 acronyms to describe electrical behavior under specific test conditions. Misreading any of them produces wrong string sizing and wrong yield estimates.

STC — Standard Test Conditions

Expansion: Standard Test Conditions Definition: The IEC 61215 lab benchmark — 1,000 W/m² irradiance, 25°C cell temperature, AM 1.5 spectrum.

Every nameplate Voc, Isc, Vmp, Imp, and Pmax defaults to STC. Cells almost never operate at STC in the field. The STC glossary entry covers the full IEC test spec.

NOCT — Nominal Operating Cell Temperature

Expansion: Nominal Operating Cell Temperature Definition: Cell temperature when ambient is 20°C, irradiance is 800 W/m², wind is 1 m/s, and back of module is open to ambient.

Modern modules report NOCT between 43°C and 48°C. NOCT-rated Pmax is typically 25–28% below STC Pmax for the same panel. See the NOCT glossary entry.

PTC — PVUSA Test Conditions

Expansion: PVUSA Test Conditions Definition: A US rating standard at 1,000 W/m² POA, 20°C ambient, 1 m/s wind, cell temperature derived from NOCT — closer to real-world output than STC.

PTC ratings typically run 8–12% below STC Pmax. CEC (California Energy Commission) uses PTC for its module database. For comparison detail see our blog on CEC vs NOCT vs PTC solar panel ratings.

CEC — California Energy Commission

Expansion: California Energy Commission Definition: The state agency that maintains the CEC list — the database of all PV modules and inverters approved for the California Solar Initiative. CEC ratings use PTC conditions.

CEC compliance is required for California incentive eligibility and is widely accepted as a quality reference across US states.

Voc — Open-Circuit Voltage

Expansion: Open-Circuit Voltage Definition: The voltage across a solar cell or module’s terminals when no current flows — the maximum voltage the module can produce.

Voc is the critical value for string sizing — Voc rises in cold weather, so the lowest expected cell temperature drives max string length. A modern 144-cell module has Voc around 45 V at STC.

Isc — Short-Circuit Current

Expansion: Short-Circuit Current Definition: The current flowing when terminals are shorted (zero voltage) — the maximum current the module can produce.

Isc drives DC conductor sizing per NEC 690.8 (Isc × 1.25 × 1.25 = ampacity required).

Vmp — Voltage at Maximum Power

Expansion: Voltage at Maximum Power Definition: The operating voltage at which the module produces peak power, typically 80–82% of Voc.

Imp — Current at Maximum Power

Expansion: Current at Maximum Power Definition: The operating current at which the module produces peak power, typically 92–95% of Isc.

Pmax (or Pmp, PMPP) — Maximum Power

Expansion: Maximum Power (or Maximum Power Point) Definition: The peak DC power output of a module at its maximum power point, equal to Vmp × Imp.

A 450 W module has Pmax = 450 W at STC. The MPPT tracker in the inverter constantly hunts for this point.

TC (TCo) — Temperature Coefficient

Expansion: Temperature Coefficient (often subscripted: TCo for Voc, TCp for Pmax, TCi for Isc) Units: %/°C Definition: The fractional change in a module parameter per 1°C rise above 25°C.

Typical 2026 monocrystalline TOPCon: TC_Pmax = −0.30%/°C, TC_Voc = −0.25%/°C, TC_Isc = +0.04%/°C. A panel operating at 55°C (30°C above STC) loses 9% Pmax versus the nameplate.

NPC — Nominal Power Capacity

Expansion: Nominal Power Capacity (or Nameplate Capacity) Definition: The rated DC capacity of a system at STC, expressed in kWp or MWp.

A 10 kWp residential system has 10,000 W of installed module nameplate at STC.

Wp — Watt-peak

Expansion: Watt-peak Definition: A single unit of DC peak power under STC. 1,000 Wp = 1 kWp.

Electrical Engineering Acronyms

These are the SI units and electrical concepts that map directly to every wire, breaker, and meter on a solar project.

AC — Alternating Current

Expansion: Alternating Current Definition: Electric current that reverses direction periodically, typically 50 Hz (Europe, Asia, Africa) or 60 Hz (US, Americas).

AC is what the grid delivers and what household loads consume. See the AC alternating current glossary entry.

DC — Direct Current

Expansion: Direct Current Definition: Electric current flowing in one direction.

Solar panels produce DC. Batteries store DC. Inverters convert DC to AC. See the DC direct current glossary entry.

V — Volt

Expansion: Volt Definition: The SI unit of electric potential difference, named for Alessandro Volta.

A — Ampere

Expansion: Ampere Definition: The SI unit of electric current, named for André-Marie Ampère.

W — Watt

Expansion: Watt Definition: The SI unit of power, equal to one joule per second. P = V × I (DC) or P = V × I × cos(φ) (AC).

kW — Kilowatt

Expansion: Kilowatt Definition: 1,000 watts of instantaneous power.

kWh — Kilowatt-hour

Expansion: Kilowatt-hour Definition: Energy delivered at 1 kW for 1 hour. The standard billing unit for residential electricity.

A 10 kW solar system running at full output for 5 peak sun hours produces 50 kWh per day.

kWp — Kilowatt-peak

Expansion: Kilowatt-peak Definition: 1,000 Wp of installed DC capacity at STC. The standard sizing metric for solar PV plants.

MWp / MWh — Megawatt-peak / Megawatt-hour

Expansion: Megawatt-peak / Megawatt-hour Definition: 1,000 kWp of DC capacity, or 1,000 kWh of energy.

kV — Kilovolt

Expansion: Kilovolt Definition: 1,000 volts. Distribution circuits run at 4–35 kV. Transmission runs at 69 kV and above.

kVA — Kilovolt-ampere

Expansion: Kilovolt-ampere Definition: The apparent power in an AC circuit — V × I without accounting for power factor.

kVA ≥ kW. Transformer and inverter nameplates use kVA.

VAr — Volt-Ampere reactive

Expansion: Volt-Ampere reactive Definition: The unit of reactive power — energy that oscillates between source and load without being consumed.

Modern grid-tied inverters can produce VArs to support voltage regulation per IEEE 1547-2018.

DC/AC Ratio — DC to AC Ratio (also called ILR — Inverter Loading Ratio)

Expansion: DC to AC Ratio Definition: Ratio of installed DC kWp to AC inverter kW rating.

A 10 kWp array on an 8 kW inverter has DC/AC = 1.25. Modern utility-scale projects run 1.3–1.5; residential rooftops run 1.0–1.2. See the inverter loading ratio glossary entry.

OCPD — Overcurrent Protection Device

Expansion: Overcurrent Protection Device Definition: Fuse, circuit breaker, or other device that interrupts current above a threshold to prevent conductor damage and fire.

NEC 690.9 governs OCPD sizing for PV systems. See the overcurrent protection device glossary entry.

Inverter and MLPE Acronyms

Inverters convert DC from panels to AC for the grid. MLPE (Module-Level Power Electronics) add per-module optimization and safety. Eight acronyms cover this domain.

MPPT — Maximum Power Point Tracker

Expansion: Maximum Power Point Tracker Definition: The algorithm that continuously adjusts DC voltage to extract the peak power from a solar string under changing irradiance, temperature, and shading.

Typical residential string inverters have 2–4 MPPTs. Each MPPT controls one or more strings independently. See the MPPT glossary entry.

MLPE — Module-Level Power Electronics

Expansion: Module-Level Power Electronics Definition: Any device that converts or optimizes the DC output of a single solar module — microinverters (Enphase) or DC power optimizers (SolarEdge, Tigo).

MLPE provides per-module MPPT, monitoring, and rapid shutdown compliance. See the MLPE glossary entry.

AFCI — Arc-Fault Circuit Interrupter

Expansion: Arc-Fault Circuit Interrupter Definition: A device that detects DC arc faults in PV systems and disconnects to prevent fires. Required by NEC 690.11 since 2011 for DC circuits over 80 V on or penetrating a building.

See the AFCI glossary entry and our blog on arc fault detection in solar PV (AFCI).

GFCI / GFDI — Ground-Fault Circuit/Detection Interrupter

Expansion: Ground-Fault Circuit Interrupter / Ground-Fault Detection Interrupter Definition: A device that detects current leakage to ground and disconnects to prevent shock. Required by NEC 690.5 for PV DC circuits.

RSD — Rapid Shutdown

Expansion: Rapid Shutdown Definition: A safety system that reduces PV conductor voltage to 30 V or less within 30 seconds of an emergency stop signal — required by NEC 690.12 since 2014.

RSD compliance is the primary driver of MLPE adoption in the US residential market. See the rapid shutdown glossary entry.

GTI — Grid-Tied Inverter

Expansion: Grid-Tied Inverter Definition: An inverter designed to operate in parallel with the utility grid, exporting surplus solar energy.

CEC Efficiency — California Energy Commission Weighted Efficiency

Expansion: California Energy Commission Weighted Efficiency Definition: A weighted-average inverter efficiency calculated across 10%, 20%, 30%, 50%, 75%, and 100% of rated output — designed to reflect real-world operating conditions.

Modern string inverters report CEC efficiency between 97.0% and 98.5%. See the CEC weighted efficiency glossary entry.

Anti-Islanding

Expansion: Anti-Islanding (function name, not strictly an acronym) Definition: The protection function that disconnects a grid-tied inverter when the utility grid is de-energized — preventing back-feed onto downed lines. Governed by IEEE 1547 and UL 1741. See the anti-islanding glossary entry.

System Performance Acronyms

Once a system is built, six acronyms describe how well it actually performs.

PR — Performance Ratio

Expansion: Performance Ratio Definition: The ratio of actual AC energy output to theoretical maximum output (POA × kWp × hours), expressed as a decimal between 0 and 1.

Formula:

PR = E_AC / (POA × kWp_DC / 1,000)

A utility-scale plant with PR = 0.84 produces 84% of its theoretical output after all losses (temperature, soiling, inverter, wiring). For full method and benchmarks see our solar performance ratio guide and how to calculate solar performance ratio. Glossary detail in the performance ratio entry.

CUF — Capacity Utilization Factor

Expansion: Capacity Utilization Factor Definition: Actual annual energy output divided by theoretical maximum if the system ran at nameplate 24/7/365.

Formula:

CUF = E_annual / (kWp_DC × 8,760 hours)

A solar plant in Gujarat with 1,800 MWh annual output and 1 MWp DC has CUF = 1,800,000 / (1,000 × 8,760) = 20.5%. See the capacity factor glossary entry.

P50 — 50th Percentile Annual Energy

Expansion: P50 — Probability of Exceedance, 50% Definition: The annual energy yield that the system is expected to exceed 50% of the time over its lifetime — the median expected production.

P90 — 90th Percentile Annual Energy

Expansion: P90 — Probability of Exceedance, 90% Definition: The annual energy yield that the system will exceed 90% of the time — the conservative downside scenario used by lenders.

P90 typically sits 8–14% below P50 for a 1-year measurement. See the P50 P90 glossary entry.

P95 — 95th Percentile Annual Energy

Expansion: P95 — Probability of Exceedance, 95% Definition: The yield exceeded 95% of the time. Used for highly conservative debt sizing in non-recourse project finance.

Yield — Specific Yield

Expansion: Specific Yield (unit-normalized) Units: kWh/kWp/year Definition: Annual AC energy output per kWp of installed DC capacity.

Phoenix residential yields hit 1,700+ kWh/kWp/year. Berlin sits at 950–1,050. Specific yield is the cleanest single number for comparing system performance across geographies.

Financial Acronyms

Twelve acronyms drive every solar investment decision, from a $15,000 residential proposal to a $200M utility tender.

LCOE — Levelized Cost of Energy

Expansion: Levelized Cost of Energy Units: $/kWh or €/kWh or ₹/kWh Definition: The average cost per unit of energy over a project’s lifetime, accounting for all CAPEX, OPEX, financing, and degradation, discounted to present value.

Formula:

LCOE = Σ (CAPEX_t + OPEX_t) / (1+r)^t ÷ Σ E_t / (1+r)^t

Residential US LCOE in 2026 sits at $0.07–$0.11/kWh. Utility-scale auctions in MENA and India hit $0.025/kWh. See our deep-dive on solar LCOE by country and the LCOE glossary entry.

IRR — Internal Rate of Return

Expansion: Internal Rate of Return Units: % per year Definition: The annualized discount rate at which the net present value of a project’s cash flows equals zero.

Residential rooftop IRR ranges from 8% to 15% in mature US markets in 2026. Utility-scale solar IRR runs 7–10% (post-tax, levered). See our guide on solar NPV, IRR, and payback.

NPV — Net Present Value

Expansion: Net Present Value Units: Currency (USD, EUR, INR) Definition: The sum of all cash flows discounted to present value at a specified discount rate.

NPV > 0 means the investment beats the discount rate. NPV < 0 means it underperforms. See the net present value glossary entry.

ROI — Return on Investment

Expansion: Return on Investment Units: % (total or annualized) Definition: A simple ratio of net gain to initial cost, often expressed as total lifetime ROI for solar (250–350% over 25 years is typical).

ROI is less rigorous than IRR or NPV because it does not discount cash flows. Useful for residential customer-facing communication; not for institutional finance.

Payback Period

Definition: Time (in years) until cumulative net cash savings equal the upfront cost.

Simple payback: 5–10 years for US residential in 2026 (post-ITC expiry). For full method see our solar payback period by country guide. Glossary entry: payback period calculation.

PPA — Power Purchase Agreement

Expansion: Power Purchase Agreement Definition: A long-term contract (10–25 years) under which an off-taker purchases all electricity produced by a solar project at a pre-agreed rate.

Corporate PPAs are the dominant procurement model for commercial and industrial solar in 2026.

ITC — Investment Tax Credit

Expansion: Investment Tax Credit Definition: A US federal tax credit equal to a percentage of qualifying solar CAPEX. The commercial ITC is 30% through 2032 (Inflation Reduction Act). The 30% residential ITC expired December 31, 2025.

See the investment tax credit glossary entry.

MACRS — Modified Accelerated Cost Recovery System

Expansion: Modified Accelerated Cost Recovery System Definition: The US tax depreciation method that allows solar projects to depreciate 85% of basis over 5 years (with 100% bonus depreciation through 2022, phasing down annually).

MACRS plus ITC delivers 35–45% combined federal tax benefit for commercial solar in 2026. See the depreciation MACRS glossary entry.

SREC — Solar Renewable Energy Certificate

Expansion: Solar Renewable Energy Certificate Definition: A tradable certificate representing 1 MWh of solar generation, issued in states with Renewable Portfolio Standards. New Jersey, Massachusetts, Pennsylvania, and Maryland operate SREC markets.

REC — Renewable Energy Certificate

Expansion: Renewable Energy Certificate Definition: A tradable certificate representing 1 MWh of any renewable generation (solar, wind, hydro, biomass). Voluntary corporate buyers and compliance buyers (RPS) drive demand.

RPS — Renewable Portfolio Standard

Expansion: Renewable Portfolio Standard Definition: A state-level policy requiring utilities to source a specified percentage of electricity from renewables by a target year. California, New York, and 28 other US states operate RPS programs.

FIT — Feed-in Tariff

Expansion: Feed-in Tariff Definition: A guaranteed per-kWh rate paid for renewable electricity exported to the grid, typically over 15–25 years. Germany’s EEG launched the modern FIT model in 2000.

See the feed-in tariff glossary entry.

Project and Construction Acronyms

Ten acronyms cover the lifecycle from contract signing through commissioning.

EPC — Engineering, Procurement, and Construction

Expansion: Engineering, Procurement, and Construction Definition: A turnkey contract model where one contractor delivers a complete solar project — engineering, equipment procurement, and construction — at a fixed price and schedule.

Most utility-scale solar in 2026 uses EPC contracting. See the EPC glossary entry.

O&M — Operations and Maintenance

Expansion: Operations and Maintenance Definition: The ongoing service contract covering monitoring, preventive maintenance, corrective repairs, and reporting after commissioning.

US utility-scale O&M costs run $7–$12/kWp/year in 2026. See the O&M glossary entry.

BoS — Balance of System

Expansion: Balance of System Definition: All system components other than panels — inverters, mounting, wiring, monitoring, AC switchgear, transformers, controls.

BoS share of total CAPEX has fallen from 50% in 2015 to 35% in 2026 as modules dropped. See the balance of system glossary entry.

PV — Photovoltaic

Expansion: Photovoltaic Definition: The technology that converts sunlight directly into electricity using semiconductor materials. See the PV glossary entry.

BIPV — Building-Integrated Photovoltaics

Expansion: Building-Integrated Photovoltaics Definition: Solar modules that replace conventional building materials — roof tiles, facade glass, skylights — rather than being mounted on top.

See the BIPV glossary entry.

BAPV — Building-Applied Photovoltaics

Expansion: Building-Applied Photovoltaics Definition: Solar modules attached to a building’s existing surface without replacing the underlying material — the conventional rooftop mount.

BOM — Bill of Materials

Expansion: Bill of Materials Definition: The complete itemized list of every component needed to build the project — module model, inverter, mounting, cables, connectors, fasteners.

See the bill of materials glossary entry.

COD — Commercial Operation Date

Expansion: Commercial Operation Date Definition: The date on which a solar project is fully tested, commissioned, and delivering energy to the off-taker — the start of the revenue period.

PTO — Permission to Operate

Expansion: Permission to Operate Definition: Utility authorization to energize a grid-connected solar system. In the US, PTO is the final permit step before the system can generate revenue.

See the permission to operate glossary entry.

NTP — Notice to Proceed

Expansion: Notice to Proceed Definition: Formal authorization for the EPC contractor to begin construction, typically issued after all permits, financing, and design milestones are complete.

IFC — Issued for Construction

Expansion: Issued for Construction Definition: The final approved revision of design drawings, released for site execution. IFC drawings supersede preliminary and IFR (Issued for Review) revisions.

Grid and Interconnection Acronyms

Solar lives or dies on grid interconnection rules. Ten acronyms cover the grid side.

NEM — Net Energy Metering

Expansion: Net Energy Metering (also called Net Metering) Definition: A utility billing arrangement where customer-sited solar exports earn bill credits equal to (or close to) the retail electricity rate.

US states are migrating from full NEM (NEM 1.0/2.0) to lower-value successor tariffs (NEM 3.0 in California, net billing in Massachusetts). See the NEM glossary entry and the NEM 3 glossary entry for the California-specific rules.

BTM — Behind the Meter

Expansion: Behind the Meter Definition: Solar or storage installed on the customer side of the utility meter, serving on-site loads first and exporting surplus.

See the behind-the-meter glossary entry.

FTM — Front of the Meter

Expansion: Front of the Meter Definition: Solar or storage interconnected at the utility distribution or transmission grid — utility-scale or merchant projects selling directly to wholesale markets.

VPP — Virtual Power Plant

Expansion: Virtual Power Plant Definition: An aggregation of distributed solar+storage and demand-response resources, dispatched as a single grid asset.

Tesla, Sunrun, and Sonnen operate large residential VPPs in California, Texas, and Massachusetts in 2026.

DER — Distributed Energy Resource

Expansion: Distributed Energy Resource Definition: Any small-scale generation, storage, or controllable load connected to the distribution grid — rooftop solar, residential batteries, EV chargers, smart thermostats.

DERMS — Distributed Energy Resource Management System

Expansion: Distributed Energy Resource Management System Definition: The utility software platform that monitors, controls, and dispatches large numbers of DERs to maintain grid stability and serve markets.

IPP — Independent Power Producer

Expansion: Independent Power Producer Definition: A privately owned electricity generator that sells output under PPAs or merchant arrangements, not affiliated with a regulated utility. See the independent power producer glossary entry.

ISO — Independent System Operator

Expansion: Independent System Operator Definition: A non-profit entity that operates a regional transmission grid and wholesale electricity market. CAISO (California), ERCOT (Texas), PJM (US Mid-Atlantic), and NYISO are the largest US ISOs.

RTO — Regional Transmission Organization

Expansion: Regional Transmission Organization Definition: Functionally identical to ISO; the distinction is FERC’s regulatory categorization. PJM, MISO, and SPP are RTOs.

IEEE 1547 — Institute of Electrical and Electronics Engineers Standard 1547

Definition: The US standard for interconnection and interoperability of distributed energy resources with the electric grid. IEEE 1547-2018 governs voltage and frequency ride-through, reactive power, and communications. See the IEEE 1547 glossary entry.

UL 1741 — Underwriters Laboratories Standard 1741

Definition: The US safety standard for inverters, converters, controllers, and interconnection equipment. UL 1741 SA is the supplemental edition aligned with IEEE 1547-2018 grid-support functions.

Battery and Storage Acronyms

Storage acronyms shape every project that pairs PV with batteries — increasingly the default in NEM 3.0 states and high-tariff markets.

BESS — Battery Energy Storage System

Expansion: Battery Energy Storage System Definition: The complete storage installation — cells, BMS, inverter, thermal management, and enclosure — engineered as a single asset.

Residential BESS: 5–30 kWh. Commercial: 100 kWh–10 MWh. Utility: 10 MWh–1+ GWh.

DoD — Depth of Discharge

Expansion: Depth of Discharge Units: % Definition: The percentage of the battery’s nominal energy capacity that has been discharged in a cycle.

LFP residential batteries are typically rated for 100% usable DoD. Older lead-acid batteries lasted longest at 30–50% DoD. See the depth of discharge glossary entry.

SoC — State of Charge

Expansion: State of Charge Units: % Definition: The energy remaining in a battery as a percentage of its full capacity.

SoC = 100% means full. SoC = 0% means empty (or at the BMS-defined floor).

RTE — Round-Trip Efficiency

Expansion: Round-Trip Efficiency Units: % (DC-DC or AC-AC) Definition: The ratio of energy out to energy in over one full charge-discharge cycle.

Modern LFP BESS achieves 90–94% DC RTE and 85–90% AC RTE (including the bidirectional inverter).

LFP — Lithium Iron Phosphate

Expansion: Lithium Iron Phosphate (LiFePO4) Definition: The dominant lithium-ion chemistry for stationary storage in 2026 — lower energy density than NMC but longer cycle life (4,000–8,000 cycles) and lower fire risk.

NMC — Nickel Manganese Cobalt

Expansion: Nickel Manganese Cobalt Definition: A lithium-ion chemistry with higher energy density than LFP — dominant in EVs. Less common in stationary storage as of 2026 due to LFP’s safety and cycle-life advantages. See the NMC battery glossary entry.

BMS — Battery Management System

Expansion: Battery Management System Definition: The electronic controller that monitors cell voltages, temperatures, and currents — balancing cells and protecting against over-charge, over-discharge, and thermal runaway. See the battery management system glossary entry.

C-rate

Definition: The charge/discharge rate normalized by battery capacity. A 10 kWh battery at C/2 charges or discharges at 5 kW (taking 2 hours). At 1C, it would charge in 1 hour at 10 kW.

See the C-rate glossary entry and our blog on C-rate battery selection for solar.

EOL — End of Life

Expansion: End of Life Definition: The point at which a battery falls below a defined useful capacity threshold (typically 70–80% of original nameplate). EOL is the warranty endpoint, not failure.

Compliance and Code Acronyms

The regulatory acronyms that gate every US and international solar project.

NEC — National Electrical Code

Expansion: National Electrical Code (NFPA 70) Definition: The US standard for electrical installations. Article 690 governs PV systems; Article 705 covers interconnection. The NEC is updated every three years. NEC 2023 is the current edition; NEC 2026 is in development.

See the NEC glossary entry, NEC Article 690, and NEC Article 705.

NESC — National Electrical Safety Code

Expansion: National Electrical Safety Code Definition: The IEEE-published standard governing utility-side electrical installations — power generation, transmission, and distribution. Complements the NEC, which governs customer-side installations.

UL — Underwriters Laboratories

Expansion: Underwriters Laboratories Definition: The leading US safety certification organization. UL 1703 covers PV modules; UL 1741 covers inverters; UL 9540 covers BESS.

IEC — International Electrotechnical Commission

Expansion: International Electrotechnical Commission Definition: The global standards body for electrical and electronic systems. IEC 61215 (module qualification), IEC 61730 (module safety), and IEC 62109 (inverter safety) are core PV standards.

See IEC 61215 and IEC 61730.

ASHRAE — American Society of Heating, Refrigerating and Air-Conditioning Engineers

Expansion: American Society of Heating, Refrigerating and Air-Conditioning Engineers Definition: The standards body for building energy use. ASHRAE 90.1 governs commercial building energy efficiency. Relevant for PV+HVAC integration.

OSHA — Occupational Safety and Health Administration

Expansion: Occupational Safety and Health Administration Definition: The US federal agency enforcing worker safety. OSHA 29 CFR 1926 governs solar construction (fall protection, electrical safety, scaffolding).

NABCEP — North American Board of Certified Energy Practitioners

Expansion: North American Board of Certified Energy Practitioners Definition: The leading US solar professional certification body. NABCEP PV Installation Professional (PVIP) is the gold-standard certification. See the NABCEP certification glossary entry.

AHJ — Authority Having Jurisdiction

Expansion: Authority Having Jurisdiction Definition: The government body that reviews and approves solar permits and inspections — typically the city or county building department, sometimes the state.

See the AHJ glossary entry.

MCS — Microgeneration Certification Scheme

Expansion: Microgeneration Certification Scheme Definition: The UK quality assurance scheme for small-scale renewable installations. MCS certification is required for UK Smart Export Guarantee (SEG) participation.

GSE — Gestore dei Servizi Energetici

Expansion: Gestore dei Servizi Energetici (Italy) Definition: Italy’s state-owned energy services company that manages solar incentive programs — Scambio sul Posto (net metering), Ritiro Dedicato, and CER incentives.

Software and Design Acronyms

Acronyms that show up in solar design tools and engineering workflows.

CAD — Computer-Aided Design

Expansion: Computer-Aided Design Definition: Software for creating precise 2D drawings and 3D models. AutoCAD is the dominant CAD tool in solar engineering for plan sets and permit drawings.

See the CAD export glossary entry.

BIM — Building Information Modeling

Expansion: Building Information Modeling Definition: 3D model-based design that integrates geometry with metadata (materials, costs, energy properties). Required for many large commercial and government building projects. See the IFC BIM integration glossary entry.

GIS — Geographic Information System

Expansion: Geographic Information System Definition: Software for spatial data analysis — parcel boundaries, terrain, solar resource maps. ESRI ArcGIS and QGIS are dominant tools.

CFD — Computational Fluid Dynamics

Expansion: Computational Fluid Dynamics Definition: Numerical simulation of fluid flow — applied to wind loading on solar racking, snow accumulation on modules, and module thermal modeling.

SCADA — Supervisory Control and Data Acquisition

Expansion: Supervisory Control and Data Acquisition Definition: The real-time monitoring and control platform used on utility-scale solar plants and substations. See the SCADA solar glossary entry.

PVsyst

Definition: The leading commercial PV simulation software (proprietary, not strictly an acronym, but ubiquitous in tender documents). PVsyst is the lender-accepted standard for utility-scale yield reports.

SAM — System Advisor Model

Expansion: System Advisor Model Definition: NREL’s free open-source PV and storage simulation software. Widely used for academic research and early-stage project screening in the US.

PSH — Peak Sun Hours

Expansion: Peak Sun Hours Units: hours/day Definition: The equivalent number of hours per day of 1,000 W/m² irradiance — a simple way to express daily solar resource.

Phoenix averages 5.7 PSH. Berlin averages 2.8 PSH. See the peak sun hours glossary entry.

Master Lookup Table: All 70+ Solar Acronyms

Use this alphabetical table when reading a datasheet, tender, or contract. Categories: IRR = Irradiance, MOD = Module, ELC = Electrical, INV = Inverter, PERF = Performance, FIN = Finance, PROJ = Project, GRID = Grid, BAT = Battery, REG = Code/Regulatory, SW = Software.

That is 99 acronyms in active use across solar engineering, finance, and operations as of 2026.

How to Use This Glossary by Role

Different solar roles need different acronyms in daily work. Here is the priority list by audience.

For Solar Installers and Field Crews

Memorize first: STC, NOCT, Voc, Isc, Pmax, MPPT, AFCI, GFCI, RSD, NEC, AHJ, PTO, NTP, BoS, BOM. These show up in every datasheet, plan set, and inspection. NABCEP exams test heavily on STC vs NOCT calculations and on NEC Article 690.

For installer workflow tools and templates, see SurgePV’s resources for solar installers and our blog on common solar installation mistakes.

For Solar Sales Professionals

Memorize first: kWp, kWh, ROI, IRR, NPV, LCOE, payback period, PPA, ITC, NEM, FIT, BESS, DoD. These are the customer-facing numbers. Avoid hammering customers with technical irradiance acronyms — focus on the financial story.

See SurgePV’s solar sales professional resources and our solar proposal software for sales-ready output.

For Solar Designers and Engineers

Master all of them. Daily use: POA, PR, CUF, P50/P90, STC, NOCT, MPPT, DC/AC ratio, BoS, OCPD, AOI, IAM, NEC 690, IEC 61215. Yield modeling depends on accurate transposition from GHI/DNI/DHI to POA and correct PR benchmarking.

A modern solar software platform like SurgePV handles the GHI-to-POA transposition automatically and produces lender-grade yield reports with PR and P50/P90 breakdowns.

For Solar Channel Managers and OEMs

Memorize first: DC/AC ratio, BoS, BOM, CEC efficiency, IEC 61215, UL 1741, IEEE 1547, NEM, ITC, SREC, RPS, FIT. Channel programs depend on regulatory compliance status and tariff economics in target markets. Resources for solar channel managers and OEMs cover the full integration workflow.

For Solar Project Finance and Asset Managers

Memorize first: LCOE, IRR, NPV, ROI, payback, PPA, ITC, MACRS, SREC, REC, RPS, FIT, P50, P90, P95, CUF, PR, O&M, BESS, COD, NTP. Bankability of a project depends on these numbers being defensible and consistently calculated.

For solar finance modeling, our generation and financial tool calculates LCOE, IRR, NPV, and P90 in one workflow.

Common Acronym Confusions to Avoid

Six pairs of acronyms get mixed up in real-world solar work. Getting them right protects your design and your reputation.

kW vs kWh

kW is power. It is an instantaneous rate — how fast energy is delivered or consumed.

kWh is energy. It is power × time — total energy delivered over a period.

A 10 kW solar system produces 10 kW at peak. Over 5 peak sun hours, it produces 50 kWh. Customers confuse the two daily. Solar professionals must not.

kW vs kWp

kW is an actual operating power level (AC or DC, at any condition).

kWp is the DC nameplate rating at STC. A 10 kWp array rarely operates at 10 kW — usually 7–8 kW peak AC.

STC vs NOCT vs PTC

STC is the IEC lab benchmark (1,000 W/m², 25°C cell, AM 1.5). Nameplate values use STC.

NOCT is a realistic operating condition (800 W/m², 20°C ambient, 1 m/s wind). NOCT Pmax is typically 25–28% below STC Pmax.

PTC is closer to real US operating conditions and is used by CEC and most US incentive programs. PTC sits between NOCT and STC — typically 8–12% below STC.

PR vs CUF

PR (Performance Ratio) measures system efficiency relative to expected output given the actual weather. It isolates system losses.

CUF (Capacity Utilization Factor) measures absolute productivity relative to running at nameplate 24/7. It includes weather.

A plant can have high PR but low CUF (sunny day in cloudy climate) or low PR but acceptable CUF (poor design in very sunny climate). Use PR for engineering QA. Use CUF for revenue forecasting.

Voc vs Vmp

Voc is open-circuit voltage — what you measure with no load. Critical for cold-weather string sizing.

Vmp is operating voltage at the maximum power point. Critical for MPPT range matching.

Confusing the two underestimates max string length by 18–22% and can damage inverters in winter.

NEM vs Net Billing vs FIT

NEM (Net Energy Metering) credits exports at the retail rate, typically with bill rollover.

Net Billing credits exports at a lower export rate (often the wholesale or avoided-cost rate). California’s NEM 3.0 is technically net billing.

FIT (Feed-in Tariff) is a guaranteed fixed rate paid for all production over a long contract — historically the most generous, now mostly closed to new entrants in mature markets.

Conclusion

Solar’s acronym vocabulary will keep expanding. Hydrogen-coupled PV, agri-PV markets, virtual power plants, and grid-forming inverters each bring new abbreviations. The 99 acronyms in this guide are the working set every solar professional needs today.

Three actions:

1. Bookmark this page as your team’s working reference — and the master lookup table specifically
2. Distinguish PR from CUF, STC from NOCT, and kW from kWh in every customer document you produce — these three confusions cost money
3. For complex yield modeling, use solar design software that handles GHI-to-POA transposition, temperature derate, and PR calculation automatically — manual math is where errors hide

For deeper coverage of specific topics, see our solar design software glossary, the detailed GHI/DNI/DHI guide, and the LCOE by country analysis. For the full glossary of solar terms beyond acronyms, browse the SurgePV glossary.

External references for further study:

• NREL National Solar Radiation Database (NSRDB) — free US irradiance data and TMY files
• PVGIS (European Commission JRC) — global irradiance and PV yield calculator
• NFPA 70 — National Electrical Code (NEC) — current US wiring and safety standard
• IEEE 1547 Standard for DER Interconnection — US grid-side DER rules
• IEC 61215 PV Module Design Qualification — global module test standard
• NABCEP Certification — US solar professional credential body
• IRENA — global renewable energy intergovernmental organization with cost and capacity statistics

Frequently Asked Questions

What is the difference between GHI, DNI, DHI, and POA in solar?

GHI (Global Horizontal Irradiance) is the total sunlight on a flat horizontal surface. DNI (Direct Normal Irradiance) is the beam component measured perpendicular to the sun. DHI (Diffuse Horizontal Irradiance) is sky-scattered light on a horizontal surface. POA (Plane of Array) is the irradiance hitting the actual tilted module surface, derived from GHI, DNI, and DHI plus array geometry. POA is what panels actually convert to electricity.

What does PR mean in solar and what is a good performance ratio?

PR (Performance Ratio) is the ratio of actual AC energy output to theoretical maximum output based on POA irradiance and rated DC capacity. A good utility-scale PR sits between 0.80 and 0.86 in 2026. Residential rooftop systems typically run 0.75 to 0.82. PR below 0.72 signals shading, soiling, inverter clipping, or wiring losses that need investigation.

What is LCOE in solar and how is it calculated?

LCOE (Levelized Cost of Energy) is the average cost per kWh over a solar system’s lifetime, accounting for upfront CAPEX, ongoing OPEX, financing, and degradation. The formula divides the net present value of all costs by the net present value of all energy produced. Residential solar LCOE ranges from $0.05 to $0.09 per kWh in 2026. Utility-scale projects hit $0.025 to $0.045 per kWh.

What is the difference between IRR and NPV in solar finance?

IRR (Internal Rate of Return) is the annualized percentage return on a solar investment, expressed as a single number like 12%. NPV (Net Present Value) is the dollar value of all future cash flows discounted to today, expressed in currency like $18,000. IRR tells you the rate. NPV tells you the dollar amount. Both should be positive for a good investment.

What does STC stand for in solar panel ratings?

STC stands for Standard Test Conditions — the lab benchmark where solar panels are rated. STC uses 1,000 W/m² irradiance, 25°C cell temperature, and AM 1.5 air mass spectrum. Every datasheet Voc, Isc, Pmax, and efficiency value defaults to STC. Real-world output is 10–25% lower because operating cell temperatures sit at 45–65°C and irradiance rarely matches 1,000 W/m².

What is the difference between kW and kWp in solar?

kW (kilowatt) is an instantaneous power rating. kWp (kilowatt-peak) is the maximum DC power rating of a solar array under STC. A 10 kWp solar array means the DC nameplate is 10,000 watts at standard test conditions. Actual AC output is typically 75–85% of the DC kWp rating after temperature, inverter, and balance-of-system losses.

What is MPPT in a solar inverter?

MPPT (Maximum Power Point Tracker) is the algorithm inside an inverter that continuously adjusts DC voltage to extract the peak power from a solar string. The maximum power point shifts with irradiance, temperature, and shading. A typical residential string inverter has 2 to 4 MPPTs. Microinverters and power optimizers provide one MPPT per module.

What does BESS mean in solar and storage?

BESS (Battery Energy Storage System) is the complete battery installation that stores solar electricity for later use. A BESS includes battery cells, battery management system (BMS), bidirectional inverter, thermal management, and protection hardware. Residential BESS sizes range from 5 to 30 kWh. Commercial BESS runs 100 kWh to several MWh.

What is the difference between PR and CUF in solar?

PR (Performance Ratio) measures system efficiency — actual output versus weather-corrected expected output. CUF (Capacity Utilization Factor) measures absolute productivity — actual annual output divided by theoretical maximum if running at nameplate 24/7. PR isolates system losses from weather. CUF includes weather. A solar plant can have high PR (0.85) but low CUF (18%) because the sun doesn’t shine 24 hours.

What does AHJ mean in solar permitting?

AHJ (Authority Having Jurisdiction) is the government body that reviews and approves solar permits. In the US, AHJ is typically the city or county building department. Each AHJ has its own plan-review timeline, fee schedule, and inspection requirements. AHJ approval is required before construction starts and before utility interconnection.