- 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 ~65–95% depending on cell type, from PERC at the low end to HJT at the high end). It only matters when the rear face 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 shorten the path current travels, reducing resistive losses and shading; mainstream cells have moved from multi-busbar (MBB, roughly 9 to 12) to super-multi-busbar (SMBB, 16 or more) designs, with zero-busbar layouts now emerging.
- 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). Modern cells, including boron-doped PERC, largely have it under control: PVEL's reliability testing puts median loss under 1%, well below the 1 to 3% older panels lost, and LID-resistant cell types like TOPCon and HJT avoid it almost entirely.
- 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 22–24%, with premium back-contact panels approaching 25%.
- 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, defined in IEC 61853-2 and referenced by IEC 61215-2:2021, 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 has now been displaced by TOPCon as the dominant cell technology in new production.
- 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. Not to be confused with Small-scale Technology Certificates, the Australian rebate instrument that shares the same initials.
- 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, because 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.