Executive Summary
- BYD, Fox ESS, Fronius, Kostal, SAX Power and SMA all achieved efficiency class A in the most recent Energy Storage Inspection analysis from HTW Berlin and Aquu
- In the 10 kW class, Fox ESS scored highest with its PQ-H3-Ultra 10.0 at an SPI of 97.0%, with 95% or more required to achieve efficiency class A
- The report explored how low storage losses needed to be for charging the battery with grid power to be cost-effective: system efficiency resulting from storing grid power must be higher than 71%
The latest Energy Storage Inspection report has cited BYD, Fox ESS, Fronius, Kostal, SAX Power, and SMA as best in class.
The half-dozen vendors all achieved efficiency class A in the analysis from HTW Berlin University of Applied Science and Aquu, a photovoltaic battery systems provider, with vendors rated on the System Performance Index (SPI) to enable comparison of solar storage systems in the 5 kW and 10 kW power classes.
In the 10 kW class, Fox ESS scored highest with its PQ-H3-Ultra 10.0, at an SPI of 97.0%. RCT Power’s Power Storage DC 10.0 and Powe Battery 11.5 was second with an SPI of 96.4%, while Energy Depot, Fronius and Kostal – the latter combined with BYD – also got an A grade and an SPI above 95%. In comparison, the lowest-ranked system achieved an SPI of only 89.3% and efficiency class G. The report cited high losses in the battery system and a standby power consumption of 64W as reasons for reduced efficiency.
For the 5 kW class, SAX Power’s AC-coupled Home Plus battery system took top honours with 93.2% SPI, while among systems with hybrid inverters, SMA scored best (92.8% SPI) with its Sunny Boy Smart Energy 5.0 and Home Storage 6.5, Kostal, again combined with BYD, was third.
Among the wider findings of the report was exploring how low storage losses needed to be for charging the battery with grid power to be cost-effective. The answer: the system efficiency resulting from storing grid power must be higher than 71%. Storing grid energy in combination with dynamic electricity prices was not profitable with all battery systems, the report warned.
Fox ESS noted its record-breaking result underlined its ‘commitment to delivering high performance, reliable solutions.’ Kostal said it was ‘honoured’ to be a winner and that the SPI seal ‘creates transparency and security’, while Fronius noted the result underlined the technical quality of its energy storage system and its ‘perfect compatibility’ with the GEN24 inverter series.
“Compared to previous years, we have significantly improved the response time of the Fronius system,” the company noted. “It now reacts much faster to load changes and thus uses PV energy even more efficiently in everyday use.”
HTW Berlin and Aquu said the 2027 report will, for the first time, evaluate storage systems with a nominal output of 15 kW.
