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As the required amount of power is the main value when deciding between a 1-phase and 3-phase system, this selection can be made independently.
If you select 3.3 kW / 4.6 kW or 6 kW at the slider "Nominal power", you will be offered a single-phase system. As soon as the slider is pushed to 9.9 kW or more, the configurator suggests a three-phase system. 
Our product configurator therefore always calculates the best storage system based on the system requirements you choose.