The more significant cost to Hertz would be the hours spent charging before the vehicle is back in service
So the time spent by a Hertz employee is 1 minute to plug and then unplug the car.
And if the car isn't fully charged and someone wants to rent it, they can either wait additional 30 minutes for full charge or get not-fully-charged car immediately and charge it at a later time.
Either way, it's effectively zero time and cost to Hertz.