The latest edition of the Energy Storage Inspection, conducted in cooperation with analytics specialist aquu, evaluates the performance of residential PV battery systems using a combination of laboratory testing and simulation. Now in its ninth year, the review is intended to improve transparency in an increasingly competitive and dynamic market.
This time round, a total of 12 commercially available PV battery systems from 10 manufacturers were analysed under standardised conditions, with results assessed using the System Performance Index (SPI). This methodology combines lab-based efficiency measurements with annualised system simulations, taking into account conversion, control and standby losses, as well as energy flows at the grid connection point.
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SPI: one metric, two system classes
The SPI distinguishes between a 5 kW and a 10 kW reference case, reflecting different system sizes and use profiles. Systems are classified into efficiency classes, allowing direct comparison of overall performance rather than individual component metrics.
The inspection included products from Sax Power, SMA and Kostal with BYD in the 5 kW class, and Kostal with BYD, Energy Depot, Fox ESS, Fronius and RCT Power in the 10 kW class. All actively participating manufacturers achieved efficiency class A.
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Key results from Energy Storage Inspection 2026
• Top performance in the 10 kW class reached 97.0 percent SPI, achieved by Fox ESS, setting a new benchmark for residential storage systems
• In the 5 kW class, Sax Power led with 93.2 percent SPI, followed closely by SMA and Kostal with BYD
• 10 of 12 systems achieved efficiency classes A or B, including systems from Fox ESS, RCT Power, Energy Depot, Fronius, Kostal with BYD and SMA
• Despite this, the efficiency spread remains wide, with one independently purchased system falling into class G
• Differences between systems are driven by conversion losses, control behaviour and standby consumption, not just battery efficiency
Award winners by system class:
In the 5 kW class, SMA led among DC-coupled systems, while Sax Power took first place among AC-coupled designs. In the 10 kW class, Fox ESS topped the DC-coupled category, with Kostal and BYD leading among AC-coupled systems.
Top performers in the 10 kW class:
Fox ESS achieved the highest system efficiency with an SPI of 97.0 percent, followed by RCT Power and Energy Depot. Fronius and Kostal with BYD also ranked among the top tier, with most systems in this class achieving efficiency class A.
Efficiency spread across systems:
The results show a wide performance range across systems. Battery efficiency reached up to 97.1 percent for Fox ESS, while inverter efficiency peaked at 97.8 percent for RCT Power. Weaker systems showed significantly lower values, along with longer settling times and higher standby consumption.
Top performers in the 5 kW class:
Sax Power ranked first with an SPI of 93.2 percent, followed closely by SMA and Kostal with BYD. The leading systems in this class are closely grouped, indicating a high level of competition in smaller residential storage systems.
The results underline how system architecture continues to play a decisive role. Both AC-coupled and DC-coupled designs are represented among the top performers, with hybrid inverter systems achieving particularly high efficiencies, notably Fox ESS (E1), RCT Power (F1) and Energy Depot (G1) in the 10 kW class.
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Usable capacity vs nameplate
Laboratory tests show that declared storage capacity does not always match what is actually usable. In more than half of the systems analysed, measured capacity exceeded manufacturer specifications, indicating that systems can, in practice, access a slightly larger share of their storage than stated limits may suggest.
The inspection further examined the economic operation of storage systems under dynamic electricity tariffs. Here, charging batteries from the grid can be beneficial under certain conditions, but only where sufficient price spreads exist and systems are equipped with intelligent energy management and forecasting capabilities.
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Warranty conditions in the lab
The report also looked at warranty conditions across more than 20 manufacturers, revealing wide variation in coverage, performance guarantees and claim requirements, complicating direct comparisons between systems.
Overall, the Energy Storage Inspection 2026 shows a market that is improving, but where performance still depends heavily on how well systems are configured, integrated and controlled in the real world. (TF)
You can read and ownload the full report from HTW Berlin and aquu here.
