I'm not sure how to tell, but I didn't notice an increase in my electric bill this month
I guess it would be impossible to really put a number to it without some kind of instrumentation, or installing a separate meter. Since it's cooling off outside a little my AC is not running as much, so maybe that offset the Volt charge added?
BTW, after seeing the long the list of retired vehicles you have in your garage I deleted two of my older cars so my posts wouldn't take up so much room, lol!