The Senec case and the discussion about the safety of PV storage systems

The drama surrounding Senec took its course at the beginning of 2022: within two months, three solar power storage systems from the Leipzig-based manufacturer burned down in their owners' homes. In Bodnegg near Ravensburg, the battery explosion pushed several windows and doors outwards and lifted the roof truss. Firefighters wearing breathing apparatus heaved the smouldering storage unit out of the cellar and into the open air, where it ignited again (see picture above). "Fortunately, there were no people in the building at the time of the detonation, so no one was injured," says the fire department's incident report.

A few days after the fire in Bodnegg, Senec responded with a radical measure: the EnBW subsidiary simply switched off thousands of its customers' solar storage units via the Internet. It later reactivated the devices, but limited the storage capacity. Numerous customers refused to accept this and sued Senec dealers for a refund of the purchase price. Many of these lawsuits were successful.

Despite the remote enforcement measures, more Senec storage units burned down. In 2023, the company then announced its plan to replace the battery modules with nickel-manganese-cobalt (NMC) or nickel-cobalt-aluminum (NCA) cells with new modules with lithium iron phosphate (LFP) cells for a total of around 100,000 storage systems delivered. The replacement campaign began in July 2024. According to Senec, 43,000 storage systems had been converted by the beginning of January 2025, with the remainder to follow by the summer. The costs of the campaign are "in the low three-digit million euro range", as a Senec spokeswoman said when asked by c't.

The fires and the costly replacement campaign have unsettled many owners of home storage units and prospective buyers: Are the devices dangerous? And should LFP cells be preferred?

In general, there have been an increasing number of reports of solar storage tank fires in recent years. Not only Senec products were affected, but also those from other manufacturers. However, in order to assess the risk, the number of such cases must be considered in relation to the number of storage tanks installed.

This is exactly what researchers at RWTH Aachen University have done. As official statistics generally do not contain precise information on the cause of fires, they systematically searched the internet for reports on storage tank fires. According to these, there were reports of six such incidents in Germany in 2022. From the beginning of 2023 to November 2023, there were 32 cases, and 37.5 for the whole of 2023. The researchers assume that most storage tank fires were also made public in 2023 due to the greater media attention given to the issue.

Just as risky as tumble dryers

The researchers divided the figure of 37.5 by 768,295, as this is the average number of home storage systems installed in Germany in 2023 according to the Federal Network Agency's market master data register. As a result, the researchers put the probability of a home storage system catching fire at 0.0049% per year.

The researchers also used similar methods to calculate the fire probabilities of other products. For tumble dryers, the risk of 0.0037 percent is similar to that of PV storage systems. For electric cars, the researchers arrived at 0.021 percent, and for cars with combustion engines at 0.089 percent. At 0.28 percent, the general risk of a house fire is around 50 times higher than with a home storage system, they emphasize in their study, which was published in the autumn.

In addition to the RWTH researchers, Ralf Haselhuhn from the German Society for Solar Energy (DGS) has also collected reports on storage fires. In his statistics, the engineer and chairman of the DGS expert committee on photovoltaics comes up with 18 cases in 2022, 49 in 2023 and 57 in 2024. Unlike the RWTH researchers, he did not just research on the internet, he says in an interview with c't. "I also get information from other sources, for example directly from fire departments and storage operators."

Haselhuhn emphasizes that it is not always clear whether the storage unit ignited or another device in the same room started the fire. Installation and usage errors are also conceivable. For example, people may install the storage units in niches or clutter them with junk, thus impairing cooling. "But in the majority of cases, the storage unit was probably the cause," says Haselhuhn.

However, even if the DGS expert's higher case numbers are taken as a basis, the probability of a storage unit fire remains low: Haselhuhn's analysis comes to 0.0097% for the year 2024. "Fortunately, the risk is extremely low," he emphasizes.

LFP storage systems can also burn

Opinions differ on the question of whether LFP storage systems should always be preferred. Storage manufacturer Sonnen has relied exclusively on LFP from the outset, citing the safety benefits of the technology as one of the reasons: NMC or NCA cells heat up to over 700 degrees in the event of a simulated short circuit, meaning that the fire can spread to other cells, the company writes on its website. "What follows is a fire that cannot be extinguished, as the oxygen for combustion is already contained in the battery material and it therefore burns even under water." LFP batteries, on the other hand, have "neither burned nor reached critical temperatures" in such tests.

The manufacturer E3/DC mainly uses LFP cells, but also NMC cells. The decisive factor is the expertise of the cell and module manufacturer, the company emphasized when asked by c't. "Our years of intensive battery monitoring at more than 151,000 domestic power plants do not show any clear advantages of a particular cell chemistry, but confirm our approach of quality-oriented supplier selection." However, the LFP share is expected to continue to grow from the current 80 percent due to a variety of factors such as quality, availability, price-performance ratio and long-term stability.

DGS expert Haselhuhn also sees no reason to favor LFP in principle. The risk of such cells exploding or burning may be lower, but is also present. The majority of fire incidents in his statistics relate to storage systems with LFP cells, which are more widespread on the market. "It is crucial that manufacturers do their homework when it comes to safety."

Haselhuhn advises prospective buyers to check whether the products meet standards such as VDE-AR-E 2510-50, DIN EN IEC 62619 and DIN EN IEC 63056 as well as the requirements of the EU Battery Regulation and whether the manufacturers can prove how they have tested them. Internal manufacturer tests are worth less than tests carried out by testing organizations. The DGS has also developed technical rules for the choice of installation location and installation by tradesmen.

(cwo)