Key figures for power storage units. This is what to look out for.

Do you consider buying a PV system with a power storage unit? Or do you want to retrofit your existing PV system with a power storage unit? Then watch out for these key figures.

System Performance Index (SPI)

The Berlin University of Applied Sciences (HTW Berlin) has evaluated the energy efficiency of solar power storage systems since 2017. The research team has developed the System Performance Index (SPI). It is an efficiency indicator that allows comparing power storage systems on both technical and economical terms. Manufacturers tend to publish very different parameters which make direct comparison difficult. The SPI indicator reveals what matters for the system owner: the economic efficiency of the storage system.

Cost savings achieved by the power storage system are set against the theoretical savings potential. With the SPI, it is easier to identify if a supposedly low-cost power storage system will ultimately become an expensive storage system after losses have been taken into account. Or vice versa: an initially more expensive power storage system turns out to be cheaper over time.

In 2020, the HTW has assessed the RCT Power Storage DC 6.0 inverter in conjunction with the RCT  Power Battery 11.5 in its annual Energy Storage Inspection in the category SPI (5kWp). The RCT Power storage system achieved 92.6%, which is the best SPI (5 kWp) result in this group. It also ranks in the highest Efficiency Class A. This is a novel classification that the HTW has introduced to ensure the comparability of differently sized storage systems.

Key figure: Settling time

The settling time is a significant key figure for the determination of the System Performance Index SPI. It indicates how quickly the solar power storage system can provide the required energy. Why is this important? The faster appliances can cover their electricity demand from the solar power storage system, the less energy is required from the public grid. This saves money.

The RCT Power storage has a settling time of fewer than 0.4 seconds, the lowest in the market. The worst performing power storage unit in this section of the Energy Storage Inspection 2020 had a settling time of more than 10 seconds in comparison. Systems with long settling times will consequently have to draw power from the grid more frequently. Every time a consumer is switched off, the battery will also discharge longer to the grid. These effects add up over time and will ultimately reflect in the electricity bill. 

Key figure: Standby consumption

An often underestimated but an important key figure for power storage system efficiency is the standby consumption. In residential buildings, power storage units are typically in a discharged state for 2000 to 4000 hours per year. Ideally, the standby consumption then is very low.

Key figure: Storage capacity

The storage capacity or rated capacity is another important indicator for a power storage system. It is the maximum amount of energy that can be stored during charging and is usually expressed in kilowatt hours (kWh). The storage capacity should be sufficient to allow the storage unit to cover the electricity demand for the evening and night hours. RCT Power power storage units are available in the range from 3.8 kWh to 23 kWh. It is recommended to calculate the required storage capacity based on your home circumstances.

Key figure: Charging cycles

Solar power storage units lose capacity over time. But how many times can they be charged and discharged before the loss in capacity becomes significant? The number of charging cycles before this capacity loss occurs is a key figure describing the expected service life.

Once installed, a storage system is charged and discharged 220 to 250 times a year. Over 20 years, this will add up to about 5000 cycles. RCT Power storage systems use Lithium iron phosphate based batteries have high cycle stability and long service life.

Conclusion: Check the key figures of power storage units before acquisition

Key figures are an important tool to assess the expected quality and service life of power storage units. The System Performance Index (SPI) is particularly significant. It incorporates various key figures to create a rating that is used to compare the efficiency and profitability of different energy storage systems.

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