I have the Victron battery management system on board and Ben is spot on. I find that if it's a coolish evening, the fridge hasn't drawn a single amp overnight sometimes. For one night stops you can work it as hard as you like. There's more than enough battery for that. I did find that running both fridge and freezer on the ferry crossing to Santander the battery was getting a bit low. But it's warm in there and the windows were closed so any warm you pull from the fridge obviously raises the surrounding temp. For the way back, I set the freezer to -18 and switched it off. It was still around -10 when we got off at the other end. Nice big thermal mass in a well insulated box. If my fridge is starting to empty, I just fill it with bottles of water that will cool as I am driving.