Hey Rossco. It's a battery destroyer mate! It's okay to use when the car is off, Andy and I did a midnight munchies cookup at Dalhousie for a couple of hours and his battery was okay but you would not want to be doing that if you are not driving every day.
It's perfect to chuck your meal in it when you are driving and a bit before meal time and it's ready when you get to where you're going!