HTW Berlin 2025 energy storage inspection: Kostal, SAX Power set new efficiency records

The Solar Storage Systems Research Group at Berlin University of Applied Sciences (HTW Berlin) has reported results of its annual energy storagy inspection and confirmed two new efficiency records.

A total of 17 manufacturers with 22 energy storage systems took part in the established energy efficiency comparison. Eight devices were new to the test, which is run annually since 2018, including those from Fox ESS, Fronius, Kostal and SAX Power.

The inspection, which is considered Europe’s key study on the topic of system efficiency for PV storage systems, found that the Plenticore G3 hybrid inverter from Kostal achieved an unprecedented 98.2% efficiency in the charging mode. Meanwhile, the Home Plus battery system from SAX Power posted a new efficiency record breaking the 98% hurdle for average efficiency in discharge mode.

“As part of the power storage inspection, we have already compared and evaluated over 90 photovoltaic battery systems since 2018. In our latest power storage comparison, a battery system with multi-level technology is represented for the first time,” Johannes Weniger, expert for solar power storage at HTW Berlin, referring to SAX Power’s product. 

The researchers explain that what distinguishes the SAX Power Home Plus system from conventional battery and hybrid inverters is that each battery cell string is equipped with its own circuit breaker. The 24 battery cell strings installed in the device can therefore be individually switched on and off in a fraction of a second.

Compared to conventional inverters, the multi-level concept enables lower conversion losses during charging and discharging. “Across the entire operating range, the AC-coupled SAX Power Home Plus battery system achieves the highest efficiencies determined to date in the power storage inspection,” the reserchers reported.

In contrast to AC-coupled battery systems, in DC-coupled systems all power electronic components are combined in the hybrid inverter. One of the 18 hybrid inverters tested only achieved an average efficiency of 92% on the test bench during charging, meaning that 8% of solar power is lost when charging the battery.

In this category, Plenticore G3 m 10 from Kostal stood out due to its technological innovation. “The IMS circuit board technology enables us to better dissipate the waste heat from the silicon carbide power semiconductors on the circuit board. This enabled us to achieve a higher power density in the same space. The advantage of this: The lower component temperatures of our high-performance battery connection reduce the conversion losses in our new hybrid inverter,” explains Andreas Forck, senior manager R&D PV Electronics at Kostal.

Overall, the researchers evaluated the energy efficiency of the devices in the two power classes 5 kW and 10 kW. In the evaluation with the System Performance Index (SPI), the hybrid inverters from the manufacturers RCT Power, Energy Depot, Fronius and KOSTAL defended their top position.

The researchers awarded 10 systems the highest efficiency class A. In addition to the European suppliers, which stole the limelight with outstanding system efficiencies, only the manufacturers Goodwe and Fox ESS were among the frontrunners. The HTW researchers were surprised that two companies decided to participate anonymously in the study despite very good efficiency results.

Call for more reliable data sheet information

This year, the inspection called for more reliable data sheet information.

Namely, as a rule, only the maximum efficiency of the inverters can be found in data sheets. Manufacturers determine these values ​​at different operating points under ideal test conditions.

In the 2025 power storage inspection, HTW Berlin shows how much the manufacturers’ efficiency specifications deviate from the laboratory measurements. Two anonymously participating manufacturers state a maximum inverter efficiency that is 2% too high on their data sheets.

With storage systems mainly discharged in the evening and at night to supply consumers in single-family homes, the power requirement is usually between 100 W and 150 W. An analysis of the 2025 Power Storage Inspection shows how different the discharge efficiency of hybrid inverters can be with a low power output of 100 W. A less efficient home storage system only achieves a discharge efficiency of 54%. The battery storage must therefore provide 186 W so that 100 watts reach the electrical consumers in the house.

With the highly efficient Power Storage DC 10.0 from RCT Power and Symo GEN24 12.0 Plus SC from Fronius, only 14% and 24% of the battery power is lost in the form of waste heat in the inverters, the inspection found.

“The manufacturers’ information on the energy content of the battery storage units is also too optimistic in two thirds of the cases examined. In four devices, the usable storage capacity determined in the laboratory was even more than 6% points below the data sheet value,” the researchers reported.