The data sheet of a PV module gives its electrical values under standard test conditions (STC) and, separately, at nominal operating temperature. For design you also have to read the temperature coefficients, the maximum system voltage, the maximum series fuse and the IEC 61215 and IEC 61730 standards that back the certificate. STC values are not what the module delivers in the field.
What the data sheet is and why it governs the design
The data sheet is the manufacturer document that declares, for each module model, the electrical, mechanical and safety values with which a string is designed. All the calculation that follows depends on it: how many modules go in series, which inverter accepts them, whether a fuse is needed and which conductor and connectors are used. A mistake in reading it is carried through the whole array.
In most markets the module is a product with certification requirements set by the local code. Those requirements usually take as reference IEC 61215, IEC 61701, IEC 61730-1 and IEC 61730-2 and UL 1703 for the testing of solar panels. The map of IEC standards for photovoltaic systems explains what each one tests. The sheet must be consistent with the product certificate, and it is the certificate, not the sheet, that proves the model was tested.
Conditions under which the values are measured
Data sheets carry two tables for the same module, each under different conditions. The sheet of the Jinko Tiger Neo JKM450-470N-60HL4 (2021) labels them at the foot of the table:
- STC (standard test conditions): irradiance of 1000 W/m², air mass 1.5 and cell temperature of 25 °C. It is the reference with which rated power is declared.
- NOCT (nominal operating cell temperature): irradiance of 800 W/m², air mass 1.5, ambient temperature of 20 °C and wind of 1 m/s, according to the same sheet.
Some sheets use the acronym NMOT instead of NOCT. Do not treat them as equivalent without looking at the conditions note. The list of technical changes in IEC 61215-2:2021 indicates that this edition removed the NMOT test from the IEC 61215 series. The value may therefore come from another standard or another method. The prudent approach is to compare only values declared under the same condition.
What each datum means
| Datum | What it is | What the designer uses it for |
|---|---|---|
| Pmax | Power at the maximum power point at STC | DC capacity of the array |
| Vmp and Imp | Voltage and current at that point | MPPT tracking window of the inverter |
| Voc | No-load voltage | Maximum string voltage at low temperature |
| Isc | Current with the terminals short-circuited | Current limit of the inverter and the protections |
| Efficiency | Pmax divided by the module area | Area needed for a given power |
| Power tolerance | Declared range of variation of Pmax | Expected real power of a batch |
The I-V curve that accompanies each sheet shows these points: Isc on the current axis, Voc on the voltage axis and the maximum power point at the knee of the curve. Current depends almost linearly on irradiance and voltage depends on cell temperature.
Temperature coefficients
Each sheet carries three coefficients, in percent per degree Celsius: that of Pmax, that of Voc and that of Isc. The first two are negative and that of Isc is small and positive. With them any value is corrected by a linear relationship from the 25 °C of STC. The Voc coefficient is the most important in design: when the cell is cold, voltage rises, and the maximum voltage of a string is calculated at the minimum expected temperature. That calculation is string sizing, developed in string sizing: Voc at minimum temperature; you can check it with our free PV string sizing tool. The Pmax coefficient, for its part, enters the temperature losses covered in losses of a solar system and performance ratio.
Coefficients are not the same between generations of the same manufacturer. Always take them from the sheet of the exact model you are going to install, and from its current revision.
System limits
Three data points set how far the design can go:
- Maximum system voltage. It is the open-circuit voltage of the string, corrected for temperature, that the module insulation admits. It is declared in the sheet, with the reference standard: 1000 V or 1500 V DC. Local installation codes usually require the installation nameplate to state the maximum system voltage.
- Maximum series fuse. It is the maximum overcurrent protection rating in series with the module. It is determined by the IEC 61730-2 test and IEC 60269-6 names it as the maximum protection current of the module. Whether a string carries a fuse or not depends on it; see gPV fuses and DC disconnect switches.
- Operating temperature. The Jinko sheet cited declares a range of -40 °C to +85 °C.
Other lines of the sheet
Bifacial modules. They give the front-face power at STC and, separately, a rear gain that depends on the albedo, the height and the tilt of the structure. That gain is not added to the rated power: it is estimated with the plant design.
Certificates. The sheet lists the standards the model was tested to. Check that IEC 61215 (design qualification) and IEC 61730 (safety) are listed, and that the certificate that comes with the equipment cites the same model and the same family. A sheet can group power variants of one family; the certificate must cover them.
Mechanical data. Dimensions, weight, junction box type, degree of protection and gauge and length of the output cables. The Jinko sheet cited gives 1903 × 1134 × 30 mm and 24.2 kg for that model, an IP68 junction box and 4.0 mm² cable (the selection of the cable and connectors is in solar cable and MC4 connectors). These data define the structure, the transport and the length of the strings.
Common reading errors
- Using the STC Voc instead of the Voc corrected to the minimum site temperature.
- Comparing powers from sheets that use different conditions (STC against NOCT).
- Ignoring the maximum series fuse when putting several strings in parallel.
- Taking the sheet of another revision or another variant of the same family.
- Assuming the power range in the sheet belongs to a single model, when they are variants with their own electrical values.
What we do at Tiza Group
The DC part of a PV array is defined by whoever designs it and carried out by its contractor. Our work includes the DC array tests: I-V curve, insulation resistance, polarity, Voc and Isc, and module thermography, also by drone, in Colombia or elsewhere on request. It also includes the commissioning of solar plants, with the transformer stations, the collector network, the step-up substation and the interconnection protections. For the electrical design of your project, from the single-line diagram to the grounding, our primary engineering team works remotely, worldwide, with your project information.
Related guides
- String sizing: Voc at minimum temperature
- gPV fuses and DC disconnect switches in solar systems
- Central or string inverter in solar plants: architecture
- IEC standards for photovoltaic systems: map and scope
- Protections of a solar plant
- Solar plant commissioning tests
References
- IEC 61215-1:2021 and IEC 61215-2:2021, Terrestrial photovoltaic (PV) modules: Design qualification and type approval.
- IEC 61730-1:2023, Photovoltaic (PV) module safety qualification, Part 1: Requirements for construction, and IEC 61730-2, test requirements.
- IEC 60904-3, measurement principles for terrestrial photovoltaic devices (STC reference).
- IEC 60269-6, Low-voltage fuses, Part 6: Supplementary requirements for fuse-links for the protection of solar photovoltaic energy systems.
- Jinko Solar, Tiger Neo JKM450-470N-60HL4 data sheet, 2021.
Frequently asked questions
Why is the data sheet power not what the panel delivers in the field?
Because rated power is measured at STC: 1000 W/m², air mass 1.5 and cell at 25 °C. In the field the cell runs hotter, irradiance varies and the spectrum changes. That is why data sheets include a second table at nominal operating temperature and the temperature coefficients, which allow the values to be corrected.
What does the maximum series fuse of a module mean?
It is the maximum overcurrent protection rating the module can withstand in series, determined in the IEC 61730-2 safety tests. It defines whether a string needs a fuse and what its maximum rating is, because with strings in parallel a fault can be fed by the current of the others.
Can I mix modules of different models in the same string?
It is not advisable. Installation practice, and some local codes, call for not mixing different module models in the same array to minimize reverse currents, and all the modules in a string must share the same operating current. The data in each sheet lets you check it.