Plain English

Solar Glossary

55 solar terms — from STC to temperature coefficient — defined without the jargon.

Independent · manufacturer-neutralReviewed June 2026Our methodology

55 terms

Panels

Bifaciality factor: For a bifacial panel, the ratio of the rear side's STC efficiency to the front side's, expressed as a percentage (typically ~70–90% depending on cell type). It only matters when the rear face actually sees reflected or scattered light, and real-world rear gain is always well below this lab figure because the back never receives equal irradiance.
Busbars: The thin conductive lines on a cell's surface that collect the current the fine gridlines gather and carry it to the cell interconnects. More, thinner busbars (multi-busbar / MBB designs) shorten the path current travels, reducing resistive losses and shading.
HJT (Heterojunction): Heterojunction Technology — a cell that sandwiches crystalline silicon between thin layers of amorphous silicon for excellent surface passivation. It delivers high efficiency, one of the best (closest-to-zero) temperature coefficients, very low degradation, and naturally high bifaciality, typically at a price premium.
Light-Induced Degradation (LID): An initial loss of efficiency in crystalline-silicon cells during their first hours-to-weeks of sun exposure, driven mainly by boron-oxygen complexes forming in the silicon (a legacy of oxygen from the Czochralski crystal-growth process). It typically costs roughly 1–3% of output up front; LID-resistant cell types like TOPCon and HJT largely avoid it.
Module efficiency: The share of incoming sunlight a whole panel converts to electrical power at STC, expressed as a percentage of the 1000 W/m² it receives over its area. Higher efficiency means more watts from the same roof space; mainstream modules today are roughly 20–23%.
NOCT / NMOT (Nominal Operating Cell Temperature): A more realistic test condition than STC: 800 W/m² irradiance, 20 °C ambient air, and 1 m/s wind, with the module open-rack mounted. This drives the cell to roughly 42–45 °C, so the rated power and figures quoted at NOCT/NMOT sit below STC and closer to real operation. NMOT (IEC 61215:2016) and the older NOCT are near-equivalent.
PERC: Passivated Emitter and Rear Cell — a crystalline-silicon cell design that adds a reflective passivation layer on the cell's back to bounce unabsorbed light back through the silicon, lifting efficiency over older designs. Long the mainstream standard, it is now being displaced by TOPCon and HJT.
Potential-Induced Degradation (PID): A power loss caused by stray leakage currents driven by the high voltage difference between the cells and the grounded module frame, which lets ions (notably sodium) migrate to the cell surface and degrade it. It is accelerated by high system voltage, heat, and humidity, and can cost a large share of output if untreated. PID-resistant construction mitigates it.
Power tolerance: How far an individual panel's actual power may vary from its nameplate rating. A positive-only tolerance (e.g. 0/+5 W) guarantees you never get less than the label; a ± tolerance means some panels ship under-rated.
Standard Test Conditions (STC): The fixed lab conditions every panel's headline rating is measured at: 1000 W/m² irradiance, 25 °C cell temperature, and an AM1.5 solar spectrum. It standardises comparison between panels but is far sunnier and cooler than a real roof, so STC numbers overstate everyday output.
Temperature coefficient of Pmax: How much a panel's power output changes per degree Celsius of cell temperature above (or below) the 25 °C STC reference, expressed in %/°C. It is negative — power falls as cells heat up — so a value closer to zero is better. Typical: about −0.34%/°C for older PERC, ~−0.30%/°C for TOPCon, and −0.24 to −0.27%/°C for HJT.
Temperature coefficient of Voc: How much a panel's open-circuit voltage changes per degree Celsius relative to 25 °C, in %/°C. Voltage rises as cells get colder, so on a frosty morning Voc can climb above its rated value. Installers use this to ensure a cold-weather string never exceeds the inverter's maximum input voltage.
TOPCon: Tunnel Oxide Passivated Contact — an n-type silicon cell architecture that adds an ultra-thin oxide layer plus a passivated contact to cut electrical losses at the rear. It offers higher efficiency, a gentler temperature coefficient, and lower degradation than PERC, and is now the dominant mainstream technology.

Inverters

CEC / weighted inverter efficiency: A real-world efficiency figure for inverters that averages performance across a range of load levels rather than just the single best point (peak efficiency). Because inverters rarely run flat-out, this weighted number better reflects everyday DC-to-AC conversion losses.
DC-to-AC ratio (inverter loading ratio): The ratio of an array's rated DC power to the inverter's AC output rating. It is usually set above 1 (commonly ~1.1–1.25) to keep the inverter working near its efficient range more of the day; too high a ratio increases clipping losses.
Hybrid inverter: An inverter that manages solar panels and a battery in one unit, handling DC-to-AC conversion plus battery charging and discharging, and (in many models) backup supply during a grid outage. It removes the need for a separate battery inverter.
Inverter clipping: When an array briefly produces more DC power than the inverter's AC rating can convert, the inverter caps output and the excess is lost. A small amount of clipping at peak times is often an acceptable trade-off for a deliberately oversized array that produces more in weaker light.
Maximum Power Point Tracking (MPPT): A control technique in inverters and charge controllers that continuously adjusts the array's operating voltage and current to keep it at the point that yields the most power as irradiance, temperature, and shading change. Multiple independent MPPT inputs let different roof faces or string lengths be optimised separately.
Microinverter: A small inverter mounted at each individual panel that converts that panel's DC to AC on the spot. This makes each panel independent — so shading or a fault on one barely affects the others — and adds per-panel monitoring, at higher cost than a string inverter.
Power optimiser: A module-level DC-DC device fitted to each panel that conditions its output (performing per-panel MPPT) before sending it to a central string inverter. It captures much of a microinverter's per-panel benefit while keeping a single inverter for the DC-to-AC conversion.
String inverter: A central inverter that converts the combined DC output of one or more series-wired strings of panels into AC. Cost-effective and simple, but because panels in a string share an operating point, shading or mismatch on one panel can drag down the whole string unless optimisers are added.

Batteries

C-rate: A measure of how fast a battery charges or discharges relative to its capacity: 1C means full capacity delivered in one hour, 0.5C in two hours. It links a battery's energy (kWh) to its continuous power (kW) capability.
Cycle life: The number of full charge-discharge cycles a battery can perform before its usable capacity falls to a defined threshold (often 70–80% of original). It depends heavily on chemistry, depth of discharge, and temperature, and underpins warranty throughput guarantees.
Depth of Discharge (DoD): The percentage of a battery's total capacity that is drawn down from full. The maximum recommended DoD sets how much of the rated capacity is actually usable — e.g. a 10 kWh battery rated to 90% DoD gives 9 kWh. Shallower discharges generally extend cycle life.
LiFePO4 (LFP): Lithium iron phosphate — a lithium-ion battery chemistry favoured for home storage for its long cycle life, strong thermal stability and safety, and tolerance of deep discharge, accepting a slightly lower energy density than nickel-based lithium chemistries.
Round-trip efficiency: The share of energy put into a battery that you get back out, after charging and discharging losses, expressed as a percentage. Modern lithium home batteries are typically around 90%, so roughly 10% of stored energy is lost in the cycle.
State of Charge (SoC): The battery's current charge level as a percentage of its usable capacity — the battery equivalent of a fuel gauge. It is the complement of depth of discharge at any moment.
Usable capacity: The energy a battery actually delivers in normal use, in kWh — its total (nominal) capacity multiplied by the allowed depth of discharge. This, not the nameplate capacity, is the number that matters for sizing how much of your home it can run.

Electrical

Maximum power point (Vmp / Imp): The voltage (Vmp) and current (Imp) at which a panel delivers its greatest power under given conditions; their product is the rated power. MPPT keeps the array operating at this point as conditions shift.
Maximum series fuse rating: The largest overcurrent-protection fuse rating a panel is rated to tolerate, used to protect parallel strings from reverse fault current. It is a key input when an installer designs string combiner and fusing.
Maximum system voltage: The highest DC voltage a panel is certified to withstand within a series string, commonly 1000 V or 1500 V. It caps how many panels can be wired in series, accounting for cold-weather voltage rise.
MC4 connector: The de facto industry-standard weatherproof DC connector for joining panels and array wiring, with a locking latch for a secure, IP-rated connection. Only genuinely compatible connectors should be mated, as cross-brand pairings can fail.
Open-circuit voltage (Voc): The voltage a panel or string produces with no load connected — its maximum voltage. Because it rises in cold weather, Voc (with the Voc temperature coefficient) sets the worst-case high voltage installers must keep below the inverter's limit.
Rapid shutdown: A safety function (required by code in many regions) that quickly de-energises the array's DC conductors on the roof when the system or grid is shut off, reducing shock risk for firefighters and responders. It is typically delivered by module-level electronics.
Short-circuit current (Isc): The current a panel produces when its terminals are directly connected with no load — its maximum current. It scales with irradiance and is used to size conductors, fuses, and the inverter's current limits.

System & Install

Glass-glass (dual-glass) module: A panel with tempered glass on both front and back instead of a polymer backsheet. The symmetric build improves durability, moisture and PID resistance, and supports bifacial operation, usually at extra weight.
IEC 61215: The international design-qualification and type-approval standard for PV modules: a panel passes a defined battery of stress tests (thermal cycling, humidity, mechanical load, and more) proving it survives real-world conditions. It is a baseline credibility mark, not a performance ranking.
IEC 61730: The international PV-module safety-qualification standard, covering electrical and fire safety; it includes the module's fire class/rating (often also expressed as a UL 790 Class A–C rating) needed for code compliance and insurance.
IP rating: Ingress Protection rating — a two-digit code for how well an enclosure (e.g. a junction box or inverter) resists solids and water. The first digit covers dust, the second water; IP67/IP68 indicates strong protection suitable for outdoor and immersion exposure.
Mechanical load rating: The maximum pressure a panel is certified to withstand, in pascals (Pa), given separately for front/down loads (snow) and rear/uplift loads (wind). It governs allowable mounting and spacing in high-snow or high-wind sites.
Tilt and azimuth: Tilt is the angle of the panels from horizontal; azimuth is the compass direction they face. Together they set how much sunlight the array captures across the day and year — in the northern hemisphere a roughly equator-facing (south) orientation maximises annual yield.

Performance

Annual degradation: The steady yearly decline in a panel's output after the first year, typically around 0.4–0.55% per year for quality modules. Combined with the larger first-year drop, it determines the end-of-warranty power floor.
Capacity factor: The ratio of a system's actual energy output over a period to what it would have produced running at full rated power the whole time, as a percentage. For rooftop solar it is modest (often ~10–25%) because the sun is intermittent.
Performance (power) warranty: The manufacturer's guarantee that a panel will still produce at least a stated percentage of its rated power after a given period (e.g. ≥87% at year 25), backstopping the degradation curve. It is separate from, and often longer than, the product/workmanship warranty.
Performance ratio (PR): A system-quality metric: the ratio of a system's actual AC energy output to the output it would theoretically produce at STC efficiency for the sunlight it received, expressed as a percentage. It nets out losses from heat, wiring, soiling, and conversion, so a higher PR means a tighter installation.
Product (workmanship) warranty: The warranty covering manufacturing defects in the panel itself, distinct from the performance warranty. Strong brands offer 25–30 years; the real value depends on whether labour and shipping for replacements are included.
Soiling loss: The output lost when dust, dirt, pollen, bird droppings, or snow accumulate on panel glass and block light. It varies by climate and tilt and is recovered by rain or cleaning; it is one of the loss terms a performance ratio captures.
Specific yield: Annual energy produced per unit of installed capacity, in kWh per kWp per year. It normalises output by system size, making it the fair way to compare productivity across systems and locations.

Grid & Market

Curtailment: The deliberate reduction of a system's output below what it could produce, ordered by the network — for example export limiting that caps how much a home system can push to the grid. It protects grid stability but can waste otherwise available solar energy.
Feed-in tariff (FiT): A rate paid to a system owner for surplus solar energy exported to the grid, set per kilowatt-hour. Modern export rates are usually well below the retail import price, which is why self-consumption and batteries have grown more valuable.
Frequency Control Ancillary Services (FCAS): Grid-balancing services (notably in Australia's NEM) procured by AEMO to keep system frequency near 50 Hz by rapidly injecting or absorbing power. Their split-second response makes batteries strong providers of contingency FCAS, creating a revenue stream beyond energy arbitrage.
Net metering: A billing arrangement that credits exported solar energy against the grid energy you import, effectively letting the meter run backwards. Where offered at full retail value it is highly favourable, but many markets have shifted to lower net-billing export rates.
Self-consumption: The share of a system's generated energy used on-site rather than exported. As export rates have fallen below retail prices, maximising self-consumption (often via batteries and load shifting) has become the main driver of solar savings.
Time-of-use (TOU) tariff: An electricity tariff where the per-kWh price varies by time of day (peak, shoulder, off-peak). It rewards shifting consumption — or discharging a battery — into expensive peak windows and charging during cheap periods.
Virtual power plant (VPP): A fleet of distributed home batteries (and sometimes other assets) coordinated by software to act as one larger resource, exporting or storing on cue to support the grid. Participating households are typically paid for the flexibility they provide.