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does the Powerwall use a auto transformer to create the neutral leg for 120v

Powerwalls definitely have a neutral connection. However, I don't know for sure whether they balance the neutral through an internal transformer or by electronic means in the inverter(s). Without much evidence either way, I assume there is no internal transformer.
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In the disassembled photos that I have seen, I haven't seen an inductor of the size needed to balance a neutral.

More to the point, when you have a digital power supply that continuously outputs a 120V sine wave with respect to ground/neutral, there is no need to have a load balancing inductor/transformer, as the balance arises out of the power supply control circuits automatically.

Stay safe out there,