[mindslight]

If I'm reading that article/pictures right (using Doubleclick Translate), it's claiming that when charging at 2.3kW, 5-15% of the power is going to the 12V system. So assuming 90% efficiency for the main power converter and 80% efficiency for the 12V converter, that's at least 6-20 amps of draw on the 12V bus? That seems quite high.

Premises wiring seems like a red herring. At least in the US, conductors are sized based on a maximum percentage voltage drop at rated current, which means the branch circuit losses should be similar when using either one at full capacity. (A lower current circuit for a longer time is actually going to be slightly more efficient because the feeders are fixed sizes)