Twelve photovoltaic storage systems with capacities of 5 kW or 10 kW were included in the comparison, among them several new products, all of which achieved efficiency class A. A new study – also examining efficiency losses when storing electricity from the grid – highlights the importance of highly efficient battery storage. The researchers also took a closer look at the warranty conditions of the devices.
From ESS News
Fox ESS is the new test winner in the “Energy Storage Inspection,” which the Berlin University of Applied Sciences (HTW Berlin) conducted for the first time in cooperation with Aquu. Aquu is an HTW spin-off founded by Johannes Weniger, who previously oversaw the university’s annual storage inspection.
This year, a total of twelve photovoltaic storage systems competed in four different categories. The evaluation was carried out separately for 5 kW and 10 kW products, with further differentiation between AC- and DC-coupled systems. To ensure comparability, the results were assessed using the System Performance Index (SPI).
Fox ESS’s 10 kW DC system achieved a new record with an SPI of 97%. According to the test results published on Thursday, this made it 3.5 times more efficient than the lowest-performing system in the comparison. The storage system, equipped with the 10 kW hybrid inverter “PQ-H3-Ultra-10.0,” impressed the testers with a very high average efficiency of 97.6% and low standby power consumption of just 4 watts.
Test winner – 10 kW category, Storage Inspection 2026
However, storage systems from RCT Power, Energy Depot, Fronius, and Kostal (DC) in combination with BYD also achieved SPI scores above 95%, earning efficiency class A. The Kostal AC system combined with BYD narrowly missed this threshold with 94.3% and was assigned efficiency class B.
Two other tested systems performed significantly worse, achieving SPIs of 91.9% (efficiency class D) and 89.3% (efficiency class G). Their manufacturers opted for anonymized publication of the results. The testers attributed the weaker performance of one system to high losses in the battery storage unit and a large standby power consumption of 64 watts, which negatively affected overall system efficiency.
Test winner – 5 kW category, Storage Inspection 2026
In the 5 kW category, the inspection again differentiated between DC- and AC-coupled systems. SAX Power’s multi-level system took the lead in this class with an SPI of 93.2%, making it the test winner among AC devices.
Among systems with hybrid inverters, SMA once again achieved the highest score, reaching 92.8% with its “Sunny Boy Smart Energy 5.0” paired with the “Home Storage 6.5” battery. Close behind was the Kostal and BYD combination, tested for the first time, with an SPI of 92.7%.
For next year, the testers announced the introduction of a 15 kW storage inspection. Manufacturers offering battery systems with storage capacities between 16 and 24 kWh will be able to participate.
“The registration phase is now open: manufacturers of AC- and DC-coupled storage systems with battery and hybrid inverters can register immediately to participate in the new 15 kW storage test,” said Johannes Weniger, founder and CEO of Aquu.
However, the researchers are not only expanding the scope in terms of performance classes. Last year they examined energy management systems for the first time, and this year the scientists investigated how low storage losses must be for charging batteries with grid electricity—such as under dynamic tariffs—to be financially viable.
According to Weniger, the decisive factor is conversion losses. The researchers illustrated this with a typical winter scenario in which a battery is charged overnight with grid electricity at €0.25/kWh and the stored energy is used in the evening when electricity costs €0.35/kWh.
“In this case, storing grid electricity is only advantageous for the household if inverter and battery losses are below 29%,” the authors of the Storage Inspection explained. In other words, the overall system efficiency must exceed 71% for the strategy to be worthwhile.
The test results showed that not all battery storage systems reach this threshold. “The price difference between charging and discharging periods is often not large enough to compensate for the losses of the battery system,” explained Nico Orth, head of the electricity storage inspection at HTW Berlin.
The study provides further details on the maximum permissible conversion losses under different electricity price scenarios.
In addition to efficiency, the testers also examined the warranty conditions of 20 well-known manufacturers, revealing some significant differences. For example, the guaranteed remaining battery capacity during the warranty period ranged from 60% to 85%, with higher values being more advantageous for storage system operators.
However, unpleasant surprises can arise in the event of damage, making it important to carefully review the fine print. “Favorable warranty conditions can often be recognized by the fact that the manufacturer does not impose extensive documentation requirements and covers the cost of replacement in the event of a defect,” said Nico Orth.
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