While extremely rare, a recent lawsuit by Walmart against SolarCity (Tesla) demonstrates that solar panel fires are a possibility.
On a sunny day, the solar panels on the roof of your home might be generating thousands of watts of power. By itself, this isn’t a risk. After all, there’s already a fat cable that runs from a utility pole or underground conduit into your home that carries up to a couple hundred amps of electricity.
Update: November 6, 2019
Walmart and Tesla have settled their lawsuit. They released the following joint statement:
“Walmart and Tesla are pleased to have resolved the issues raised by Walmart concerning the Tesla solar installations at Walmart stores. Safety is a top priority for each company and with the concerns being addressed, we both look forward to a safe re-energization of our sustainable energy systems.”
By itself, large amounts of electricity isn’t a fire risk. Your house doesn’t burn down when you flip on the toaster or turn on your electric clothes dryer.
An electric arc is like a mini bolt of lightning. It happens when a spark from damaged wiring or faulty equipment travels through the air into nearby objects. When arcing happens in the walls of your home or near anything combustible, fires can ignite.
How do solar panels catch on fire?
Just like the 44,000 electrical fires that happen every year, a solar panel fire is the result of a malfunction. A properly designed and installed solar panel system with correctly operating equipment presents no fire hazard to your home.
But when solar panel equipment is poorly installed, in some cases this can result in electrical faults that cause arcing. Arcing can ignite the encapsulant layers that surround the solar cells in a panel, or the backsheet at the rear of the panel. Both are made of plastics and are flammable.
What happened with the SolarCity panels at Walmart?
The Walmart lawsuit, filed on August 20th 2019, accused that SolarCity (now Tesla) practiced “widespread, systemic negligence” and “failed to abide by prudent industry practices in installing, operating and maintaining its solar systems.”
The court filing claims that Tesla’s own inspectors identified multiple issues with their own installations but failed to repair them. According to court documents, the issues include:
• improper wire management, including abraded and hanging wires;
• inadequate wire connecting practices and poor grounding;
• inaccurate as-built drawings; and
• solar panel modules that were broken or contained dangerous hotspots.
We should wait for further facts to come out during the court proceedings, but at this point it seems as if Walmart might have a case here.
Should you not install solar panels because of a fire risk?
If you read the court filing in depth, you’ll find well-documented instances of substandard work by SolarCity, such as stepping on solar panels and not properly handling cables, resulting in damaged wire insulation.
There are millions of solar panels installed every year, and fires that are caused by solar panels are extremely rare - so rare that data on the number of incidents is hard to find.
The best report I could find is from the United Kingdom , where there are nearly one million PV systems currently deployed. A report by the UK government found that a total of 58 fires have ever been found to be caused directly by PV systems, an incident rate of 0.0061%.
That’s a really low number. You should probably worry more about your toaster catching fire than your solar panels.
Precautions you can take
Still, there are some lessons to learn from the Walmart lawsuit, and some steps you can take to basically reduce the fire hazard to zero.
If you read my guide on how solar installers work you’ll know that there are three major types of solar inverters: central inverters, power optimizer-based systems, and microinverters.
There are pros and cons to all of these, and a good solar installer will recommend one that balances costs with performance.
But one of the benefits of microinverters is that it mitigates one of the causes of solar panel fires. This is because a microinverter converts DC power to AC power directly at each solar panel, resulting in AC power traveling down from your roof to your basement electric panel instead of DC power.
Having AC power moving across your roof is safer because it lessens the danger of electric arcs.
Why is DC power more prone to arcing?
Alternating current repeatedly switches polarity from positive to negative. This is the type of power supplied to the electrical sockets in your home. Here in North America, our AC power switches 60 times per second (60 hertz).
While it is possible for AC power to arc, when AC power switches from positive to negative and vice versa, the current will briefly hit zero. When this happens, the arc is quenched. Because of this behavior of AC power, it requires higher voltages to sustain dangerous arcing.
Direct current, on the other does, doesn’t switch polarity. Instead, it’s a sustained stream of electrical current. In comparison to rapidly switching AC power that is quenched 60 times a second, DC acts more like a firehose of electricity. This characteristic of DC power means that sustained arcing occurs much more easily.
Here’s a video that dramatically shows the different behavior between AC and DC power at the same voltages. Notice the charring on the switch after the DC demonstration.
Why arcing can occur with a solar panel system
Solar panels generate DC power. In a system that doesn’t use microinverters, the DC power travels along wiring to a central inverter some distance away. If the insulation on wiring is damaged anywhere along that path, electricity can arc from the wire to metallic objects such as the aluminium frame of solar panels.
As you can see from the video above, DC power at high voltage causes energetic arcing that can easily ignite nearby flammable materials.
Here’s another relevant paragraph from the Walmart court filing:
78. A Walmart contractor (not Tesla) called the Yuba City fire to Walmart's attention after observing signs of a recent fire on the store's roof. The contractor took photos showing that
wires were still sparking at the time of discovery, indicating that the fire had occurred recently. Photographs also revealed that arcing had affected numerous wires at the Yuba City site over an extended period of time, degrading the wires' insulation and resulting in the incineration of a substantial section ofthe store's roof. The extent ofthe damage to the wires indicated that the fire was sufficiently severe that it could have burned the entire store to the ground. Because of Tesla's failure to provide a final root cause analysis, Walmart has no assurance that the next store to experience a Tesla-caused fire will be so fortunate.
Here’s a photo from the same court filing:
Evidence of a fire that appears to have been caused by arcing from solar panel wiring. (Credit: Walmart)
While damage to solar panel wires can happen because of other reasons - such as squirrels chewing on them - the occurrence of seven fires at installations by the same installer does suggest that substandard work could be to blame.
How do microinverters help prevent fires?
As mentioned earlier, microinverters lessen the risk of electric arcing. Each panel has its own microinverter that is bolted to the rear. DC power from the panel only has to travel a few inches to the microinverter, which converts the DC power to AC, which flows the rest of the way down your roof to the electric panel.
This means that very little DC power is flowing on your roof. It’s still possible for wiring damage to occur in the few inches between the panel and the inverters (such as by a pesky squirrel) but it’s a much lower probability.
But a string inverter is probably going to be located in your basement, which means that there is a much longer distance of wiring that carries DC power.
This is also true with power optimizers because they are DC-to-DC converters. Power optimizers still require a central inverter, which means that you have DC power flowing across your rooftop.
Hot spots are another fire hazard
One other fire risk with solar panels has nothing to do with the difference between DC and AC power. Instead, a phenomenon called a hot spot, in very rare cases, can generate so much heat that it causes flammable materials in or near the panel to catch on fire.
A hot spot occurs when a single cell is damaged or is unable to generate electricity while nearby cells continue to generate power. Due to the way that solar panels are wired, a dead cell can end up carrying the current of neighboring cells, causing it to heat up.
In extreme cases a fire can result. Fortunately, all solar panels that you install on your home will have multiple devices called bypass diodes that prevent that the heating effects related to a non-producing solar cell.
There are a number of things that can prevent a single solar cell in a panel to stop generating power. Physical damage is one possibility, such as poor handling by the installer that causes the glass or cell to crack. (The Walmart court filing documented a SolarCity employee stepping on a panel.)
It can also occur due to shading by something as simple as leaf falling on a panel, or bird poop.
Obviously, if a solar panel caught on fire every time that a leaf fell on it or a bird left droppings on your roof, that would not be a very useful product. So, it should be emphasized that all solar panels that are permitted to be installed on your home are designed to safely deal with hot spots. This is one of the things that is part of California Energy Commission certification.
In the United States, any solar panel that would be installed on your roof will have CEC certification. You can check the manufacturer’s datasheet to be certain.
Preventing hot spots
Even though hot spots are nearly always mitigated by the bypass diodes in a solar panel, this is one of the things that can be prevented with periodic maintenance. Here are some steps you can take:
Monitor your system for individual panel issues. If you have microinverters or power optimizers, you will have the ability to check the power output of individual panels in your system. Make a habit of looking at your power output during the day at least once a week. If you notice that any solar panel has an abnormal drop in output, this could mean that you have a shading issue. Get a ladder and take a look, and see if you have dirty or cracked panel.
Clean any debris on your panels. In most cases, it isn’t necessary or even recommended to regularly clean your solar panels. Unless you live in a dusty and dry region, periodic rain may be all the cleaning you need. However, you should still regularly take a look up at your system and look for issues such as debris. If you have a soiled panel that isn’t getting naturally cleaned by rain or wind, you should clean it off to help prevent a hot spot issue. If you don’t feel comfortable on a ladder, pay a professional for the service.
Maintain nearby foliage. Not only can nearby trees shade your system and reduce your power harvest, but deciduous trees can drop leaves on your system every fall. If you can, keep your trees trimmed to prevent this annoyance.
Bottom line: solar panel systems are extremely safe
The takeaway from this article is that solar panel systems are very safe, with extraordinarily low rates of fires - an incident rate in the neighborhood of 0.0061% according to a study in the United Kingdom.
That said, a negligent contractor can cause all kinds of problems for you, and fire hazards are just one. If you are considering installing a solar system, the best thing you can do is educate yourself so that you know the right questions to ask when interviewing an installer. Read our complete guide to buying solar for your home to learn lots of handy tips.
If you’re still concerned, think about choosing microinverters. Not only do they mitigate DC arcing, but they also give you panel-level monitoring of your system so that you can identify issues such as hot spots. String inverters don’t give you this capability.