I-V curve importance in Roof Top system

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By looking at the I-V curve it shall be possible to know about the optimum power to be delivered by the SPV panel/array/source.The peak hour behavior of current and voltage can also be estimated from the curve.

A module which is rated at 17 volts will produce less than its rated power when used in a battery system. This is because the working voltage will be between 12 and 15 volts. As wattage (power) is the product of volts times amps, the module output will be reduced.

Example: A 50 watt module working at 13.0 volts will produce 39.0 watts (13.0 volts x 3.0 amps = 39.0 watts).This is important to remember when sizing a PV system. An I-V curve is simply all of a module’s possible operating points, (voltage/current combinations) at a given cell temperature and light.

Many installers will simply put a voltmeter on the leads from a module to test open-circuit voltage. If that Voc is within the manufacturers specifications, the installer will feel confident the entire module is fine. A single or even multiple bad cells in that module however might not cause a decrease in Voc. By doing an I-V curve analysis with a field I-V curve analyzer, you will be able to see the entire curve with voltage and current measurements at numerous points, the degradation of the panel performance will be readily visible, and that panel can be deemed defective by the installer and set aside rather than installed.

Below are two I-V curves taken from the same panel. The first curve was taken in full sunlight with no shading on the panel. You can see a nice smooth curve with an open circuit voltage of 23.5V. Simply applying a voltmeter to the leads would have read approximately this same Voc value. The second curve was taken with two cells blocked, simulating bad cells or bad electrical connections between a couple of cells. You can see how badly the I-V curve has deteriorated, indicating problems with the panel. However the Voc still measures 23.2V. If a voltmeter was used to field test this panel rather than an I-V curve analyzer, this problem would have been missed.

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