Solar module datasheets explained: Pmax, Voc, Isc, Vmp, Imp, temperature coefficients, NMOT
A module datasheet describes one current–voltage curve measured under standard test conditions, plus the numbers that say how that curve moves with temperature and light. Read the rows together and compare modules on efficiency and temperature behaviour, not on the wattage in the model name.
Standard test conditions
The electrical data on a datasheet is measured at standard test conditions (STC): 1000 W/m² irradiance, 25 °C cell temperature and the AM 1.5 reference spectrum of IEC 60904-3. STC is a laboratory reference, not a forecast: in sunshine cells run well above 25 °C, so outdoor power is lower.
The five electrical rows
- Isc, short-circuit current: the current at zero volts, the most the module makes; it rises in proportion to irradiance.
- Voc, open-circuit voltage: the voltage at zero current, the highest the module makes.
- Vmp and Imp: voltage and current at the maximum power point, where the inverter's tracker holds the module.
- Pmax: the power at that point, so Pmax = Vmp × Imp.
The rows check each other. Canadian Solar's TOPBiHiKu6 at 640 W gives Vmp 42 V and Imp 15.26 A, which multiply to 641 W, a match within rounding. A column that doesn't multiply out deserves a question: on the JinkoSolar Tiger Neo 54HL4-B sheet, the 420 W column repeats the 415 W column's Vmp (31.95 V), Imp (12.99 A) and efficiency (21.25%), and 31.95 × 12.99 is 415 W.
| Row | Canadian Solar TOPBiHiKu6, 640 W | Polycab G2G TOPCon, 605 W | Emmvee E530-540HCM144-B, 540 W |
|---|---|---|---|
| Vmp × Imp | 42 V × 15.26 A | 43.12 V × 14.03 A | 41.54 V × 13 A |
| Voc and Isc | 49.4 V, 16.32 A | 51.79 V, 14.79 A | 49.5 V, 13.62 A |
| Module efficiency | 23.7% | 23.42% | 20.9% |
| Temperature coefficient of Pmax | −0.29 %/°C | −0.30 %/°C | −0.35 %/°C |
| NMOT or NOCT | 41 °C, NMOT | 45 °C, NOCT | 45 °C, NOCT |
Efficiency and power classes
Module efficiency is Pmax divided by the light falling on the module at STC. The TOPBiHiKu6 measures 2382 × 1134 mm, 2.70 m², which receives 2701 W at 1000 W/m², and 640 ÷ 2701 is 23.7%. Wattage grows with area, so a 640 W, 132-cell module and the 375 W, 120-cell Vikram Solar Prexos are not rivals on watts: compare efficiency and price per watt.
One datasheet covers a family of power classes, sorted by flash test: Polycab's G2G sheet lists six, from 580 W to 605 W. Compare like classes, or the class you will be supplied. Power tolerance, 0 to +10 W for the TOPBiHiKu6 and 0 to +5 W for the G2G, means every module measured at or above its class; measurement uncertainty, which Canadian Solar states as ±3%, is separate.
Temperature coefficients
Each coefficient is the change per degree of cell temperature away from 25 °C. The Pmax coefficient drives yield: at a cell temperature of 65 °C, 40 degrees above STC, a module at −0.29 %/°C loses about 11.6% of its power and one at −0.35 %/°C about 14%. On our comparison pages the modules listed as TOPCon give −0.29 to −0.30 %/°C and those listed as mono PERC −0.35 %/°C.
The Voc coefficient is negative too, −0.25 to −0.28 %/°C on our pages, so voltage rises in the cold and string design uses the coldest expected cell temperature. For the TOPBiHiKu6, 49.4 V at −0.25 %/°C becomes about 53.7 V at −10 °C, 35 degrees below STC: 49.4 × (1 + 0.0025 × 35). Against its 1500 V maximum system voltage that allows at most 27 modules in series, before the inverter's input limit and local codes. The Isc coefficient is small and positive, +0.04 to +0.05 %/°C.
NMOT, NOCT and power at NMOT
NMOT, the nominal module operating temperature in IEC 61215 since its 2016 edition, is the temperature a module reaches at 800 W/m², 20 °C ambient and 1 m/s wind, mounted on an open rack and working at its maximum power point. NOCT, from earlier editions, uses the same irradiance, ambient and wind but leaves the module open-circuited. A module delivering power turns part of the light into electricity rather than heat, so it runs cooler: the two are not interchangeable. Canadian Solar gives an NMOT of 41 °C; Polycab, Emmvee and Vikram give a NOCT of 45 °C, Waaree 43 °C. Our pages put both in one row, "NMOT or NOCT", so check the label on the sheet.
Power at NMOT is lower than at STC because the light is weaker and the cells warmer: 484 W against 640 W for the TOPBiHiKu6, about 76%.
System voltage, warranty and certification rows
Check the maximum system voltage (1000 V for the Tiger Neo 54HL4-B, 1500 V for the other modules on our pages). Warranties come in two parts: product, 12 years on the four sheets here that give one, and power, 25 to 30 years, with 1% to 2% first-year degradation. IEC 61215 covers design qualification and IEC 61730 safety; IEC 61701 (salt mist) and IEC 62716 (ammonia) matter near the coast and on farms.
Verify against the manufacturer's datasheet. The figures here come from the datasheets behind our comparison pages, each linked from its page. Datasheets are revised, so check every number against the manufacturer's current datasheet before you design a string or order.
FAQ
What is the difference between STC and NMOT?
STC is the laboratory reference: 1000 W/m², 25 °C cell temperature and the AM 1.5 spectrum. NMOT is the temperature a working module reaches at 800 W/m², 20 °C ambient and 1 m/s wind, and power at NMOT is nearer everyday output.
Why does Voc matter for string design?
Voc rises as the module gets colder. The string's Voc at the coldest expected cell temperature must stay below the module's maximum system voltage and the inverter's maximum input voltage.
Is a higher-wattage module always better?
No. Wattage grows with module size, so compare efficiency, the temperature coefficient of Pmax, price per watt, and whether the size and weight suit the roof or structure.