All Herrmans e-bike front lights for 6-12V deliver the same light output on both 6V and 12V. The electrical power is also practically the same for both voltages. H-Black Pro E 6-12V electrical power is 5W with both 6V and 12V. The current at 6V is about 0,8A and at 12V about 0,4A.
All Herrmans dynamo and e-bike rear lights are equipped with a rectifier, so you don’t need to pay attention to the polarity of the cables.