Solar PV Glossary: Definitions for Panels, Inverters, and System Terms

Plain-English definitions for the terms that turn up on spec sheets, in installer contracts, and across our own guides. If a term is missing or a definition feels thin, email editorial@accelerate-solar.com and we'll add it.

Electrical Spec Sheet Terms

MPPT
Maximum Power Point Tracking. An inverter algorithm that continuously adjusts the operating voltage of a panel string so it produces the most power achievable given current sunlight and temperature. Modern string inverters carry 1-3 independent MPPT inputs; each runs its own tracker, which is why you can put an east-facing string on one MPPT and a south-facing string on another without dragging both down to the weaker side.
Voc
Open-circuit voltage. The voltage a panel produces with no load connected, measured at Standard Test Conditions (STC). Spec sheets list it as Voc (STC). String sizing math uses Voc plus a temperature coefficient, in cold weather Voc rises, and a string designed only for the room-temperature value can exceed the inverter's maximum input voltage on the first cold morning.
Vmp
Voltage at maximum power. The operating voltage at which a panel delivers its rated power at STC. Vmp is lower than Voc because the panel is under load. MPPT trackers aim to hold each string at Vmp; if shading or mismatch pushes a panel away from Vmp, output drops disproportionately to the voltage shift.
Isc
Short-circuit current. The current a panel produces with its terminals shorted, at STC. It's the upper bound for string fusing and combiner-box current ratings. Like Voc, Isc has a temperature coefficient, usually small (+0.04 to +0.06%/degC), but it matters for hot-climate sizing.
Imp
Current at maximum power. The current delivered when a panel sits at Vmp under STC. Datasheet Pmax equals Vmp x Imp.

Performance and Test Conditions

STC
Standard Test Conditions. The lab benchmark used for every datasheet rating: 1000 W/m^2 irradiance, 25 degC cell temperature, AM1.5 spectrum. STC almost never matches a real roof, in mid-afternoon sun a panel's cell temperature is closer to 55-65 degC, so real output usually trails the STC rating by 10-20%.
NMOT
Nominal Module Operating Temperature. The cell temperature a panel reaches under more realistic conditions: 800 W/m^2 irradiance, 20 degC ambient, 1 m/s wind. The NMOT rating gives a fairer point-of-comparison than STC for real-world output.
PR
Performance Ratio. The ratio between a system's actual energy output and the theoretical output that the panels would deliver if they ran at STC for the same irradiance. Healthy residential rooftops sit at 0.78-0.85 PR; commercial flat-roof installs with good ventilation can reach 0.85-0.90. A PR below 0.75 usually means shading, soiling, or an inverter sizing problem.
kWp
Kilowatt-peak. The STC-rated power capacity of an installed system. A "5 kWp system" delivers 5 kW at STC, not 5 kW continuously. Annual energy is closer to kWp x peak sun hours x PR.
AC/DC ratio
The ratio of installed panel capacity (DC) to inverter output capacity (AC). A 7.5 kW DC panel array on a 6 kW AC inverter is a 1.25 AC/DC ratio. Ratios of 1.1-1.3 are common because peak DC output rarely happens at full irradiance, clipping a few peak hours per year costs less than oversizing the inverter.

Power Electronics

MLPE
Module-Level Power Electronics. Catch-all for hardware attached to individual panels rather than a central inverter, microinverters (Enphase IQ8A) and DC power optimizers (SolarEdge P370, Tigo TS4-A-O). MLPE isolates mismatch losses to the individual panel, which is the main argument for it on shaded or complex roofs.
String inverter
Central inverter that converts the DC output of a series-connected string of panels to AC. Cheaper per watt than MLPE. Single point of failure for the whole string; one weak panel drags the entire string's output.
Microinverter
A small inverter mounted under each panel that converts that panel's DC directly to AC. Each panel runs independently. Higher cost per watt than string inverters, longer warranties (typically 25 years), and the simpler retrofit path because there's no high-voltage DC running on the roof.
Power optimizer
DC-DC converter mounted under each panel that conditions the panel's output before sending DC down the string to a central inverter. Solves the mismatch problem of plain string inverters without going full microinverter on cost. SolarEdge and Tigo are the two volume manufacturers.
Rapid Shutdown
A NEC 690.12 requirement: residential and commercial PV systems must be able to drop DC voltage on the roof to under 30 V within 30 seconds of an emergency shutdown. MLPE systems meet rapid shutdown intrinsically; string-inverter installs add module-level rapid-shutdown devices to comply.

Panel Technology

PERC
Passivated Emitter and Rear Contact. The dominant residential panel cell architecture from roughly 2017-2023. P-type silicon; degrades around 0.45-0.55%/yr after first-year light-induced degradation. PVEL scorecards consistently show PERC fleet medians slightly worse than n-type alternatives but still well within useful-life targets.
TOPCon
Tunnel Oxide Passivated Contact. N-type silicon cell with a thin tunnel-oxide layer that reduces carrier recombination at the rear contact. Lower light-induced degradation than PERC (around 0.35-0.45%/yr after year one), better high-temperature performance, marginally higher efficiency. LONGi Hi-MO X6 is a popular TOPCon line.
HJT
Heterojunction Technology. N-type crystalline silicon sandwiched between amorphous silicon passivation layers. The lowest degradation rates in the market (0.25-0.35%/yr) and the best high-temperature coefficient. Costlier than PERC and TOPCon; REC Alpha Pure and Panasonic EverVolt are HJT lines.
BIPV
Building-Integrated Photovoltaics. Solar generation hardware that replaces a conventional building material rather than attaching to it. Tesla Solar Roof is the consumer-facing example; commercial BIPV facades and skylights are more common. Always carries a per-watt premium over conventional panels because the product also has to function as a roof or cladding material.
Bifacial
Panels with photovoltaic cells on both the front and rear surfaces, so they can convert light reflected off the ground or roof in addition to direct sunlight. Bifacial gain (the extra output relative to a monofacial panel of the same Wp) is typically 5-15% on ground-mount installs over reflective surfaces, lower on dark rooftop installs.

Codes, Standards, and Certifications

IEC 61215
International standard for crystalline silicon panel design qualification and type approval. Defines the mechanical-load, thermal-cycling, hail-impact, and damp-heat tests that a panel must pass to claim a 25-year design life. Reputable panels list IEC 61215 certification on their datasheet; cheap import panels often don't.
IEC 61730
International standard for panel safety qualification. Covers electrical insulation, partial discharge, hot-spot endurance, and impulse voltage tests. Complementary to IEC 61215, 61215 is about durability, 61730 is about safety.
IEC 62109
International standard for the safety of grid-tied PV inverters. Covers isolation, fault protection, and grid-fault response. Any inverter sold for grid-tied residential or commercial use will list IEC 62109 -1 (general) and -2 (specific) compliance.
NEC 690
Article 690 of the US National Electrical Code, covering solar photovoltaic systems. Defines required conductor sizing, overcurrent protection, grounding, rapid shutdown (NEC 690.12), and labelling. NEC editions are adopted state by state; the 2023 cycle is now active in most jurisdictions.
NABCEP
North American Board of Certified Energy Practitioners. The independent body that certifies PV installers (PVIP), commissioners (PVCS), and designers (PVDS) in the US and Canada. The NABCEP PVIP credential is the closest thing residential solar has to a third-party trade certification; the public lookup at nabcep.org lets you verify any installer's status before signing a contract.
PVEL
PV Evolution Labs (now part of Kiwa). Independent reliability test lab that publishes the annual PVEL Reliability Scorecard. Their thresher-tested data is the closest thing the market has to honest fleet-level reliability comparison between manufacturers.

Financial and Incentive Terms

ITC
Investment Tax Credit. The US federal Residential Clean Energy Credit currently lets homeowners deduct 30% of an owned solar system's installed cost from their federal income tax liability. Non-refundable; if your tax bill is smaller than the credit, the balance carries forward. Applies to systems placed in service before December 31, 2025 at 30%, then steps down.
DSIRE
Database of State Incentives for Renewables and Efficiency. The canonical free database for US state-by-state and utility-by-utility solar incentives, run by NC State University. Always check DSIRE before ruling out an incentive program, state programs come and go quickly.
PPA
Power Purchase Agreement. A financing model where a third party owns the panels installed on your roof and sells you the electricity they produce at a contracted rate (usually with an annual escalator). The third party claims the ITC and depreciation; the homeowner pays a monthly bill in exchange for not owning the equipment.
SREC
Solar Renewable Energy Certificate. A tradable certificate awarded for each MWh of solar electricity generated, in states that operate an SREC market (NJ, MA, DC, MD, OH, PA, IL, DE). System owners can sell SRECs into the market for cash income on top of any electricity savings. Prices range from $5 to over $300 per certificate depending on the state and year.
Net metering
A utility billing arrangement where solar generation pushed back to the grid is credited at (or near) the retail electricity rate. The strongest financial mechanism for residential solar, where it exists in its full form. Many utilities have moved to lower-credit successor arrangements (NEM 3.0 in California, "net billing" in others) that pay closer to the wholesale rate.

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