Bifaciality factor
For a bifacial panel, the rear side's STC efficiency expressed as a percentage of the front side's, typically 70 to 90% depending on cell type. It only pays off when the rear face actually catches reflected or scattered light, and the real-world gain always sits below this lab figure because the back of a panel never receives the same irradiance as the front.
Busbars
The thin conductive lines running across a cell's surface that collect the current gathered by the finer gridlines and carry it to the cell interconnects. More, thinner busbars, the multi-busbar or MBB design, shorten the distance current has to travel, cutting resistive loss and reducing how much of the cell they shade.
HJT (Heterojunction)
Heterojunction Technology: a cell that sandwiches crystalline silicon between thin layers of amorphous silicon for strong surface passivation. It delivers high efficiency, one of the closest-to-zero temperature coefficients available, very low degradation, and naturally high bifaciality, usually at a price premium over other architectures.
Light-Induced Degradation (LID)
An upfront efficiency loss in crystalline-silicon cells during their first hours to weeks in the sun, caused mainly by boron-oxygen complexes forming in the silicon, a side effect of oxygen left over from the Czochralski crystal-growth process. It typically costs 1 to 3% of output before the panel settles. LID-resistant architectures such as TOPCon and HJT are largely immune to it.
Module efficiency
The share of incoming sunlight a whole panel converts into electrical power at STC, expressed as a percentage of the 1000 W/m² landing on its surface. A higher figure means more watts from the same roof area; mainstream panels today sit around 20 to 23%.
NOCT / NMOT (Nominal Operating Cell Temperature)
A second, more realistic test point alongside STC: 800 W/m² irradiance, 20°C ambient air, and a 1 m/s breeze, with the panel open-rack mounted. Those conditions push the cell to roughly 42 to 45°C, so power figures quoted at NOCT/NMOT sit below the STC number and track closer to how a panel actually performs. NMOT (IEC 61215:2016) and the older NOCT test are near-equivalent.
PERC
Passivated Emitter and Rear Cell: a crystalline-silicon design that adds a reflective layer to the back of the cell, bouncing unabsorbed light back through the silicon and lifting efficiency over older designs. It was the mainstream standard for years and is now being displaced by TOPCon and HJT.
Potential-Induced Degradation (PID)
A power loss driven by stray leakage current between the cells and the grounded module frame, which lets ions (sodium in particular) migrate to the cell surface and degrade it. Higher system voltage, heat, and humidity all accelerate it, and an untreated panel can lose a large share of its output. PID-resistant construction is the standard mitigation.
Power tolerance
How far an individual panel's actual output can vary from its nameplate rating. A positive-only tolerance, such as 0/+5 W, guarantees you never receive less than the label states; a plus-or-minus tolerance means some panels ship under their rated power.
Standard Test Conditions (STC)
The fixed lab benchmark every panel's headline power rating is measured against: 1000 W/m² of irradiance, a 25°C cell temperature, and an AM1.5 solar spectrum. STC makes panels comparable to each other, but the conditions are sunnier and cooler than most real roofs ever see, so the nameplate number overstates everyday output.
Temperature coefficient of Pmax
How much a panel's power output moves for every degree Celsius the cell runs above or below the 25°C STC reference, in %/°C. It's always negative: power falls as cells heat up, so a figure closer to zero is the better outcome. Typical values run about −0.34%/°C for older PERC cells, around −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 shifts per degree Celsius away from the 25°C reference, in %/°C. Voltage climbs as cells get colder, so a string can read well above its rated voltage on a frosty morning. Installers use this figure to keep a cold-weather string under the inverter's maximum input voltage.
TOPCon
Tunnel Oxide Passivated Contact: an n-type silicon architecture that adds an ultra-thin oxide layer and a passivated contact to cut electrical losses at the rear of the cell. It runs more efficient, degrades slower, and carries a gentler temperature coefficient than PERC, and is now the dominant mainstream technology.
CEC / weighted inverter efficiency
A real-world efficiency figure for inverters, averaged across a spread of load levels rather than taken at a single best-case point (peak efficiency). Since inverters rarely run flat-out, this weighted number is the more honest read on everyday DC-to-AC conversion loss.
DC-to-AC ratio (inverter loading ratio)
The ratio of an array's rated DC power to the inverter's rated AC output. It's usually set above 1, commonly around 1.1 to 1.25, to keep the inverter working in its efficient range for more of the day; push the ratio too high and clipping losses start to climb.
Hybrid inverter
An inverter that manages both solar panels and a battery in one unit: DC-to-AC conversion, battery charging and discharging, and, on many models, backup supply during a grid outage. It removes the need for a separate battery inverter alongside the solar one.
Inverter clipping
What happens when an array briefly produces more DC power than the inverter's AC rating can convert: the inverter caps its output and the surplus is lost. Some clipping at peak sun is often an accepted trade-off for deliberately oversizing an array to lift output in weaker light.
Maximum Power Point Tracking (MPPT)
A control method built into inverters and charge controllers that continuously adjusts the array's operating voltage and current to land on whichever point yields the most power as irradiance, temperature and shading shift through the day. Multiple independent MPPT inputs let different roof faces or string lengths be tracked separately rather than forced to share one operating point.
Microinverter
A small inverter fitted to each individual panel that converts that panel's DC to AC on the spot. Every panel becomes electrically independent, so shading or a fault on one has little effect on the rest, and per-panel monitoring comes as standard, at a higher cost than a string inverter.
Power optimiser
A module-level DC-to-DC device fitted to each panel that runs its own per-panel MPPT before handing the conditioned output to a central string inverter. It captures much of the per-panel benefit a microinverter offers while still using a single inverter for the actual DC-to-AC conversion.
String inverter
A central inverter that takes the combined DC output of one or more series-wired panel strings and converts it to AC. Simple and cost-effective, but because every panel in a string shares one operating point, shading or a mismatched panel can drag the whole string down unless power optimisers are added.
C-rate
A measure of how fast a battery charges or discharges relative to its capacity: 1C delivers the full capacity in one hour, 0.5C in two. It's the link between a battery's stored energy (kWh) and its continuous power capability (kW).
Cycle life
The number of full charge-discharge cycles a battery can complete before its usable capacity falls to a defined threshold, often 70 to 80% of the original figure. It depends heavily on chemistry, depth of discharge and operating temperature, and it's what underpins a warranty's throughput guarantee.
Depth of Discharge (DoD)
The percentage of a battery's total capacity drawn down from full. The maximum recommended DoD sets how much of the rated capacity is actually usable: a 10 kWh battery rated to 90% DoD gives you 9 kWh in practice. Shallower discharges generally stretch cycle life further.
LiFePO4 (LFP)
Lithium iron phosphate: the lithium-ion chemistry favoured for home storage for its long cycle life, thermal stability, safety, and tolerance of deep discharge, at a slightly lower energy density than nickel-based lithium chemistries.
Round-trip efficiency
The share of energy you get back out of a battery for every unit you put in, after charging and discharging losses, as a percentage. Modern lithium home batteries typically sit around 90%, meaning roughly a tenth of stored energy is lost over the full cycle.
State of Charge (SoC)
A battery's current charge level as a percentage of usable capacity: the fuel-gauge equivalent for a battery. At any given moment it's the mirror image of depth of discharge.
Usable capacity
The energy a battery actually delivers in normal use, in kWh: its nominal capacity multiplied by the allowed depth of discharge. This is the figure that matters for sizing how much of a home it can run, not the nameplate capacity.
Maximum power point (Vmp / Imp)
The voltage (Vmp) and current (Imp) at which a panel delivers its greatest power under given conditions; multiply the two together and you get the rated power. MPPT is the mechanism that keeps an array sitting at this point as conditions change.
Maximum series fuse rating
The largest overcurrent-protection fuse a panel is rated to tolerate, used to protect parallel strings against reverse fault current. It's one of the inputs an installer works from when designing a string combiner and its 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 sets the ceiling on how many panels can be wired in series once cold-weather voltage rise is accounted for.
MC4 connector
The de facto industry-standard weatherproof DC connector for joining panels and array wiring, with a locking latch that keeps the connection secure and IP-rated. Only genuinely compatible connectors should ever be mated; cross-brand pairings can fail.
Open-circuit voltage (Voc)
The voltage a panel or string produces with nothing connected to draw current: its maximum possible voltage. Because Voc climbs in cold weather, it, together with the Voc temperature coefficient, sets the worst-case high voltage an installer has to keep under the inverter's ceiling.
Rapid shutdown
A safety function, required by code in many regions, that quickly de-energises an array's rooftop DC conductors when the system or the grid is shut off, cutting shock risk for firefighters and other first responders. It's typically delivered through module-level electronics.
Short-circuit current (Isc)
The current a panel produces when its terminals are joined directly with no load: its maximum possible current. It scales with irradiance and is one of the figures used to size conductors, fuses and an inverter's current limits.
Glass-glass (dual-glass) module
A panel built with tempered glass on both the front and the back, instead of a polymer backsheet on the rear. The symmetric construction improves durability and moisture and PID resistance and supports bifacial operation, usually at the cost of extra weight.
IEC 61215
The international design-qualification and type-approval standard for PV modules. Passing it means a panel has been through a defined battery of stress tests, thermal cycling, humidity, mechanical load and more, proving it survives real-world conditions. It's a baseline credibility mark, not a performance ranking.
IEC 61730
The international PV-module safety-qualification standard, covering electrical and fire safety. It includes a module's fire class or rating, often also expressed as a UL 790 Class A to C rating, which building code compliance and insurance can both depend on.
IP rating
Ingress Protection rating: a two-digit code for how well an enclosure, a junction box or inverter for instance, resists solids and water. The first digit covers dust, the second covers water; IP67 or IP68 signals strong protection suited to outdoor use and even brief immersion.
Mechanical load rating
The maximum pressure a panel is certified to withstand, in pascals, stated separately for front-facing/downward loads (snow) and rear-facing/uplift loads (wind). It governs what mounting and spacing is allowable at high-snow or high-wind sites.
Tilt and azimuth
Tilt is the panel angle measured from horizontal; azimuth is the compass direction the panels face. Together they set how much sun an array captures across the day and the year; in the northern hemisphere, an orientation facing roughly toward the equator (south) maximises annual yield.
Annual degradation
The steady year-on-year decline in a panel's output after its first year in service, typically 0.4 to 0.55% a year for a quality module. Combined with the larger first-year drop, this is what sets the power floor a manufacturer guarantees at the end of the warranty term.
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 entire time, as a percentage. For rooftop solar it's modest, often 10 to 25%, simply because sunlight is intermittent.
Performance (power) warranty
The manufacturer's guarantee that a panel will still produce at least a stated share of its rated power after a given period, 87% or more at year 25 is a common benchmark, which backstops the degradation curve. It's a separate, and usually longer, promise than the product/workmanship warranty.
Performance ratio (PR)
A system-quality metric: the ratio of a system's actual AC energy output to what it would theoretically produce at STC efficiency given the sunlight it received, as a percentage. It nets out losses from heat, wiring, soiling and conversion, so a higher PR points to a tighter installation.
Product (workmanship) warranty
The warranty covering manufacturing defects in the panel itself, distinct from the performance warranty. Strong brands offer 25 to 30 years, though the real value of that promise depends on whether labour and shipping for any replacement are actually included.
Soiling loss
Output lost when dust, dirt, pollen, bird droppings or snow build up on panel glass and block light. It varies with climate and tilt and clears with rain or cleaning; it's 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. Normalising output by system size this way is what makes it possible to fairly compare productivity across different systems and locations.
Curtailment
A deliberate reduction of a system's output below what it could produce, ordered by the network, export limiting that caps how much a home system can push to the grid is one example. It protects grid stability but can waste solar energy that was otherwise there for the taking.
Feed-in tariff (FiT)
The rate a system owner is paid for surplus solar energy exported to the grid, set per kilowatt-hour. Modern export rates typically sit well below the retail import price, which is exactly why self-consumption and batteries have become more valuable over time.
Frequency Control Ancillary Services (FCAS)
Grid-balancing services, notably in Australia's NEM, procured by AEMO to hold system frequency near 50 Hz by rapidly injecting or absorbing power. Their split-second response makes batteries strong providers of contingency FCAS, opening a revenue stream beyond simple energy arbitrage.
Net metering
A billing arrangement that credits exported solar energy against the grid energy imported, effectively letting the meter run backwards. Where it's offered at full retail value it's a strong deal, though many markets have since 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 with a battery and some load shifting, has become the main lever for solar savings.
Time-of-use (TOU) tariff
An electricity tariff where the per-kWh price changes with the time of day, peak, shoulder, off-peak. It rewards shifting consumption, or discharging a battery, into the expensive peak windows and charging back up during the cheap ones.
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 contribute.