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Module Measurement

Safety First

  • Do not measure a module when it is installed in an array. The voltage is up to 1000 V and deadly.
  • Only measure one module at a time. Do not measure modules connected together.
  • Check that the module voltage is below 50 V by consulting the label.
  • Modules get hot in sunlight. Be careful about hot surfaces and/or wear gloves.

The accurate measurement of a module is quite challenging. Special testers give the standard test conditions: 1000 W/m2 of sunlight, 25 °C and spectrum of Airmass 1.5 (AM 1.5). The guides on the following pages are to give an indication that the module is functional. Further tests would be needed to ensure that the module is performing to specification.

Interpretation of a module label

photovoltaic module label
A module label shows the specifications of a module under standard test conditions. It is usually on the back of the module. Standards vary slightly throughout the world but most labels will follow this format.

Modules from major manufacturers are measured according to standards. A typical module label is shown above. Measurements are at STC unless otherwise stated.

Solar Module Type
Usually looking up the module type will give a more detailed data sheet on the module. In this case, we can tell its a 295 W module and there are 60 cells in the module. As there are 60 cells it is more likely to be used in residential applications than a large centralized power station.
Maximum Power (Pmax)
295 watts is the power of the module under STC at the maximum power point. In the field, the module will likely be at a higher temperature so the actual power will be lower.
Power Tolerance
Under STC the module might have a power up to 3% higher. A higher power is not always a good thing as it could overload the power electronics.
Maximum Power Voltage (VMP)
When operating at Pmax the voltage is 32.4 volts.
Maximum Power Current (ISC)
When operating at Pmax the current is 9.1 amps. Note that 32.4 V × 9.1 A = 295 W.
Open Circuit Voltage (VOC)
At STC the no-load voltage is 39.7 volts. This is the voltage that would be measured on a sunny day when the light intensity is close to 1000 W/m2 and the temperature is 25 °C.
Maximum Power Current (ISC)
When the leads of the panel are shorted the current is 9.61 amps.
Nominal Operating Cell Temperature (NOCT)
In the field the module will operate at 45°C under the specific conditions of NOCT, which is different from STC. NOCT has Irradiance 800 W/m2, air temperature or 20°C, wind velocity of 1 m/s and an open back. 

The rest of the details cover how the module is installed and that it passes safety standards.

STC
STC – Standard test conditions. The label finally notes that the standard test conditions are 1000 W/m2 of sunlight, 25 °C and spectrum of Airmass 1.5 (AM 1.5). All modern modules are tested using these conditions.

The label usually does not give the fill factor but we can calculate it. In this case it is (32.4 × 9.1)/(39.7 × 9.61) = 0.772. Most modules will have a fill factor from 0.7 to 0.8.