Umm, that is literally exactly what you said: "Level 2 charging is 24 amps total, 12 amps on each 120VAC line"

The two 120 V A/C lines don't run at 12 amps each. When you measure one wire against the other, the total amperage is 12, not 24.

Watts = volts x amps

Therefore, if you simply double the voltage, you double the watts.

Energy transferred = watts x time (usually expressed as kilowatt hours).

So, if you double the voltage and leave the amperage the same, the amount of time it will take to transfer a given amount of energy (such as that required to charge the Volt's battery) will be halved.

My understanding is that this is exactly what happens if you use the OEM EVSE plugged into 240V instead of 120V. It will take 6 hr to give the Volt a full charge - half the 12 hr it takes at 120 V/12 amps. The Volt is *capable* of charging at 16 amps but the OEM EVSE is limited to 12 amps. So charging with an aftermarket 240 V EVSE results in faster charging (4.5 hr) because those EVSEs can charge at 16 amps.

It should be ~6 hr on the oem evse at 240 V and 12 amps, and it is a bit more than 4 hr on an aftermarket level 2 evse at 240 V and 16 amps