Carmen was born in the middle of winter and we gets loads of snow some years, so I kept plenty of extra formula on hand. Ready-made and powdered, plus a case of bottled water. Now that she's a year old it's not such a worry, but I still keep ready-made formula in her diaper bag and some in the house just in case.
I always like to be prepared; I've been through the blizzards up north and the tropical storms down south so I know it's entirely possible to be stuck in the house with no electricity even if it's not a full-on natural disaster. I've been stuck in the car too (before Carmen was born, thankfully) so I know to keep a full emergency kit in the back seat with blankets, food, jumper cables, first aid kit and all that.