How To Find A Bad Solar Panel In A String

People often wonder what’s wrong with their solar panels if the power output drops. While there are many reasons why the power output may drop, many people focus on trying to find a bad solar panel in their string of panels.

However, with a simple-to-use tool and some safety preparation, you can test each panel if necessary. 

In this blog, we discuss:

  • How to test a solar panel
  • How to identify a ground or earth fault
  • How to determine if your solar panel is broken
Close up of a solar panel

How to find a bad solar panel in a string

When we talk about strings of solar panels, we are talking about string converters. If your solar array has a smart technology design, you can track the power output at a few different levels. Those include:

  • Total energy output for the array
  • Total energy output by a string of panels
  • Total energy output by the solar panel if equipped with a microinverter

With smart technology and microinverters, you can check each panel’s output to see if there is a bad panel or at least one with low energy output.

That’s pretty simple. With a string inverter, the entire energy production for each panel is linked, and then the energy is sent to the inverter.

If a panel is bad, the total energy produced by each panel drops, making it difficult to determine which panel is faulty.

So how do you test a solar panel string to find the bad apple in the bunch? Keep reading! 

How do you test a solar panel string?

You may or may not be able to tell which string is underperforming. If you cannot tell, You would use a voltage meter or multimeter and test each connection where the string and the inverter connect.

You will need to power down the system so that you are working with minimal power. If you are not familiar with electricity and testing, then hire an electrician or a solar contractor to do this for you. 

If your system is under warranty and it most likely is, then uses the power of that warranty to have a professional do the checking. 

See also: Solar Panels Maintenance: Essential Tips for Optimizing Efficiency and Longevity

Once you identify the underachieving string

Once you determine which string is not hitting its peak energy production, you can begin to test each panel. Here is how to do that. 

How do you know if a solar panel is bad?

Using electrical safety techniques to keep you and your solar panels safe, you would turn your attention to each solar panel. If they have inline fuses, you will check them to ensure that the fuse is not the problem. 

You know when a solar panel is bad because the power output from the solar panel is beneath its efficiency rating. So the first thing to know is this:

  • Throughout the day, solar panels produce a range of electricity; In the early morning and late afternoon, the panel will produce the least amount of power. The solar panel will produce the most energy at noon and through the hotter part of the day. So, the reading will be relatively consistent and low when we say the power output is beneath the panel’s efficiency rating. 

That information also means that you must know the range of energy production to expect from the panel. 

Using a Multimeter

You would use a multimeter or voltage meter to test each panel. Panels in the string will all have a similar output except for those that are faulty.

For example, if you begin to test each panel, and you notice that most have a voltage reading of 5.6 and then one has a reading that is either fluctuating rapidly or has a low reading of, say, 2.1, you know something is wrong with that panel. 

The next step would be to determine what the issue is. Also, be sure to test all the panels in the string as more than one panel may be faulty.

Why solar panel underperform

On the other hand, there may also not be anything wrong with the panel. Here’s a closer look at what causes panels to underperform. 

  • Shading – Shading occurs when something blocks the panel from direct sunlight
  • Dirt or debris on the panel
  • Faulty wiring from the panel to the string

Shading is a big concern. Seasonal events, such as tree growth or leaving out in the spring, can cause sudden shading issues.

If the panel is dirty, then it cannot absorb as much sunlight as it should. Follow the manufacturer’s suggestions for washing the panel. Inspect the wiring from the panel to the string and look for signs of damage. 

These issues can cause the power production of the panel to drop. 

How will you identify if a string has an earth fault?

An earthy fault is a grounding fault in the wiring. To detect a ground fault, you use an insulation resistance test, which tests the strength of the insulation material around electrical wires between point A and point B.

If there is a difference, then there is a ground fault within the wiring. 

Check the Hardware – Fuses and Circuit Breakers

 Sometimes the wiring between the solar panel and the inverter will have either an inline fuse or a circuit breaker. If a fuse blows or a circuit breaker trips, it can indicate a problem.

Both devices are wires inline between the solar panel and the inverter or controller to protect the inverter or controller from spikes in power. 

Hardware Issues Happen

If a fuse blows or a breaker trips often, then there is a severe problem with the power emission from the panel, and it could be a faulty wire insulator.

Wire insulators are the coating over electrical wires that prevents the wire from forming a ground and shuttling power to places where it can be dangerous. 

Even a tiny nick in the plastic insulation around a wire can be enough of a space to cause an earth fault.

If an earth fault is present, then replacing the wiring is necessary. However, if the ground is small, you may get away with simply splicing the wire at the ground site. 

A reminder that electricity is dangerous. If you do not know how to work with electricity, hire a professional. The life you save, maybe your own. 


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Elliot has 20+ years of experience in renewable technology, from conservation to efficient living. His passion is to help others achieve independent off-grid living.

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