Test a used solar panel by using a multimeter to compare the volts and amperage generated by the panel to what the panel should be producing. This gives an accurate assessment of how well it is performing compared to a new solar panel of the same type.
The watts produced by a solar panel are a function of its voltage and amperage, in which the voltage multiplied with the amperage equals the watts of power being output. To test the efficiency of a solar panel, first locate the converter box, which is usually placed on the back of the solar panel. Open the panel and find the positive and negative connections housed within it.
To test voltage, align the panel so it is facing full sunlight and set the multimeter to read DC volts. Use the first volt setting that is higher than what the panel should be capable of producing. Attach the positive and negative leads from the multimeter to the corresponding connections in the converter box. Turn on the multimeter and record how many volts are being generated. Once a reading has been taken, turn off the multimeter and disconnect it from the converter box.
To test amperage, turn the panel so it is getting no sunlight. Amperage is more dangerous and can cause sparks, electric shock or fire. With the panel facing away from the sunlight, turn on the multimeter to read DC amperes, then connect the positive and negative leads to the converter box connections. Once the leads are in place, turn the panel into full sunlight and take an amperage reading. Turn the panel back away from the sunlight before disconnecting the leads again.