How to Test a Solar Panel

To accurately test a solar panel you need to find the open circuit voltage and short circuit current, paying mind to the cell temperature, ambient temperature, coefficient and the angle towards the sun. Disconnect the panel from any regulator, if testing an array you will need to test each panel individually. Check the spec sheet on the back of the panel for the necessary readings applying the temperature de-rating factors.

You will need to have a multi meter with a voltmeter and an ammeter that can read at least 10 amps.

To measure the correct cell temperature, not the ambient temperature, you will need to use a thermocouple or temperature probe at the rear of the panel, not the front as the glass is an insulator, this is the true cell temperature and needs to be 25 degrees C. A panel de-rates the hotter the cell, decreasing 0.5% for every degree Celsius over 25

[actual power = [rated power x (-0.5 x cell temperature - 25)].

How you mount a panel will also affect this, make sure there is airflow under the cells.

To cool a panel you can run water down the front of the panel, this will increase power output by 20-50%

Here are some handy links that further explain how to test a panel;

Solar Panel Temperature

Solar Panel Output based on Area

Temperature Measurement

Back of Module Testing methods

Out of the factory panels are tested before being released with a flash test of 1000w/m2 at 25 degrees C. For a better understanding watch this video;

Open Circuit Voltage (voc)

To test open circuit voltage take your multimeter and set it to voltmeter and place the probes in the correct connectors and set to volts DC. The reading should be within 10% of the specs on the rear of the panel. Keeping in mind the coefficient, common readings are;

100w 12v 36 cell panel - 20-22 voc

200w 24v 72 cell panel - 40-44 voc

250w 24v 60 cell panel - 37-39 voc

Short Circuit Current (isc)

To test the short circuit current you will need to set your multimeter to amps and make sure the cables are correctly placed in the multi meter, then put the probes into the connectors paying mind to the positive and negative outputs. Reading the current in this configuration is the short circuit current, this will be within 20-35% of the panels specs, Keeping in mind the coefficient, common readings are;

100w 12v 36 cell panel - 5.6 amps isc

200w 24v 72 cell panel - 5.6 amps isc

250w 24v 60 cell panel - 8.9 amps isc

It is important to take into account the derating factor of temperature, if the cell temperature is to hot the current reading will decrease,

The VOC and ISC test is the best and most accurate way for a consumer to test a panel, if the panel is within specs the issue will lie with the regulator and/or battery.  If the panel is not within specs possible issues are;

  • Broken cables

  • Blown diodes

  • Faulty connections

  • Cell failure.

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