When we say we “test” all of our panels, we mean we test the output of the individual panels by themselves, without anything attached to them—that is, without a “load” (e.g., not connected to an inverter, charge controller, batteries, etc.). These external components are all considered “loads.” Testing panels without a load is the standard practice in the solar industry to determine whether a panel is functioning properly. This testing involves two measurements: voltage and current.

For voltage, we measure Voc (open circuit voltage). For current, we measure Isc (short circuit current, where current = amperage). At OGG, we typically skip Isc testing—not for any specific reason other than assuming that if the Voc is good, the Isc will most likely also be good (though I could be wrong about this assumption!). We may include Isc testing in the future.

Today, we encountered a situation where a customer insisted that one of our panels was “bad” because he connected it to a load and relied on the Vmp (voltage at maximum power), rather than the Voc, to assess its condition. Vmp is the voltage measured under load. The customer, who claimed to be a retired licensed electrician, argued the panel was faulty. I strongly disagree with this assessment.

The problem with his argument is that faults between the panel and other components, such as the inverter, battery, or charge controller, can cause low readings even if the panel itself is functioning properly (e.g., if the Voc is above 90%). It’s important not to fall into this misconception.

About a year ago, I sold a panel at a steep discount because a buyer made this same argument, and at the time, I didn’t know any better. Now I do.