The loses from AC/DC conversion must be pretty low. A typical solar installation with battery backup (e.g. Powerwall 2) would convert from DC to AC in the solar

The loses from AC/DC conversion must be pretty low. A typical solar installation with battery backup (e.g. Powerwall 2) would convert from DC to AC in the solar inverter, AC to DC when charging the Powerwall, DC to AC when discharging the Powerwall, then AC to DC when charging your electric car (and as you say, most likely DC to AC when driving it). The original Powerwall was DC, so they must have run the numbers and found out the impact was minimal to switch to AC.