Comparing the efficiency of residential storage systems, researchers at Berlin’s College for Technology and Economics (HTW) found the best performing system to be Kostal plus a BYD battery, with that set-up recording a system performance index (SPI) of 91.4%. RCT Power ranked second, at 90.7%, with systems provided by SMA and Sonnen taking the following positions.
The worst performing system tested had an SPI of 83.7%.
The index indicates the gap between a theoretical, perfect system of the appropriate size and the one being tested, in terms of realistic cost savings. The difference emerges due to inefficiencies intrinsic to power electronics.
The study provides the most comprehensive information on the efficiency of certain battery storage systems, although the products tested are only available in Germany and only ten of the 60 manufacturers contacted by the HTW team agreed to participate. Once the 16 storage systems produced by those companies had been tested, and the results sent to the manufacturers, the number of participants willing to have the results made public halved, leaving only five brands who were willing to be named in the study.
“The lack of transparency is absolutely inadequate,” said Thomas Seltmann, Advisor for Photovoltaics at North-Rhine Westphalia’s consumer assistance office. “Consumers must have the possibility of reviewing, somewhat, the product for which they are paying large sums.”
With that in mind, the researchers presenting the study stressed it was very likely there were numerous systems that would perform considerably worse than the system the index ranks last. “We have to see what we do not see today,” said Martin Rother, of SMA, who added the companies not listed in the ranking have a considerable market share.
Significance of the index
The SPI is an economic parameter which takes into consideration the grid consumption and grid feed-in of a household. A solar system plus battery storage enables a household to increase self-consumption to the point where electricity can be fed into the grid, providing system owners a cost advantage. The larger the losses – due to inefficiency – on the inverter and storage system are, the lesser the amount of electricity fed into the grid. The SPI is determined by simulating energy flows in a model home on the basis of provided efficiency guideline information.
Under those conditions, the winning system – the Kostal Plenticore plus 5.5 with a 10 kWh BYD battery storage system – theoretically saves its owners €1,250 per year. With an SPI of 91.4%, the actual cost reductions the system can achieve in a model home are €1,140 per year.
That means each percentage point of SPI equates to around €12 per year, said Johannes Weniger, who has been executing the research at HTW Berlin.
Based on the €12 figure, the difference in possible cost savings per year between the best and worst performing systems is €100 per year.
That equates to €1,000 of missed savings over the typical lifetime of a battery storage system. With system costs of battery storage currently in the €1,000/kWh of capacity range, the difference between the best and worst systems listed amounts to at least 10% of overall investment costs, for a 10-kWh system, and the magnitude of that figure will only rise as system costs continue to fall.
Bigger isn’t always better
Another argument which underlines the relevance of efficiency, derives from the comparison of system performance by dependency on storage capacity.
The availability of self-consumption can be a prime motivation for battery storage system buyers.
For instance, the RCT Power system boasts a usable storage capacity of around 5.3 kWh and would need grid consumption of around 2.2 kWh/year in the model home. A rival, undisclosed, system in the ranking reports a usable storage capacity 2 kWh above that of the RCT Power product, offering a temptation to buyers. But they would end up with a system that has a grid consumption 200 kWh than the RCT offering. “When buying a storage system, one should consider not only the capacity of the battery, but also its efficiency”, said Mr. Weniger.
Another potential fallacy concerns high-voltage battery systems, whose efficiency increases with the number of battery modules used, because they are serially connected. The more modules, the higher the input voltage for the battery inverter. The efficiency grades for high storage capacity are therefore better than those for smaller systems of the same production series. Thus, if the efficiency grades for smaller systems are not mentioned, buyers should ask for them.
In that vein, SMA’s Mr. Rother said his company’s Sunny Boy Storage system was tested with a 6.4 kWh battery, while Kostal’s winning system used a 10 kWh battery. Since both systems are high-voltage, it is likely the SMA system would have performed better with a 10 kWh battery, said Mr. Rother.
Additionally, the index was calculated using a model home with a 5kW PV system and annual electricity consumption of 5 MWh. However, many of today’s rooftop systems are larger, with 7 kW not a rarity and many systems reaching 10 kW – meaning the latter would be affected by additional levies under Germany’s remuneration scheme. Moreover, many households have a heat pump or use an electric vehicle. Both increase electricity consumption and power output with which the battery would typically be discharged.
For households with a smaller annual power consumption, the discharge rate of the battery – which plays the biggest role in high efficiency – is around 500 W to 1 kW. High efficiency at a low power rating is therefore essential for an high SPI. If a household had an electric vehicle being charged at 3.7 kW, the efficiency-relevant power output is set at a higher region. As a result, the HTW researchers are considering running another test for households with high electricity consumption and production.
There is one myth the study soundly busted, however. There is no way to justifiably claim DC or AC-coupled system are more efficient than their counterpart. In the early stages of battery storage systems, it was often said DC systems were more efficient because some conversion processes could be omitted. Later, it was often claimed, AC-coupled systems were better, because it was easier to optimize them for low power rated applications. Both arguments are correct in principle though there are many parameters, with the effect a clear trend cannot be identified. Though the top two systems are both DC coupled, on average both AC and DC-coupled systems produced 88% identical results from the testing.
Mr. Weniger said the tests also illustrated manufacturers learn, and systems improve, over time, particularly in the case of Kostal. Aside from the winning Plenticore system, launched this year, the manufacturer also had its Pico 6.0 system tested. That only achieved a penultimate placing in the ranking, with the product developed in 2009 and brought to market in 2012.
Christoph Kiesel, Product Manager at Kostal, took heart from the result, and said the efforts made in research and development had paid off.
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