As my refrigerator and freezer are both designed to run off of 12v, 120v (supplied by a diesel generator, if needs be) or propane, Having my food spoil during a power outage isn't at the top of my list of worries. However, having rented whole lotta apartments before moving into my current digs, I understand the stress that can come from fretting over having hundreds of dollars worth of food ruined, thanks to a blackout.
Growing up in a tornado-prone region of Canada, I was taught that, once the lights go out, the fridge and freezer stayed closed: every time you open either of them when there's no juice to run the refrigerator's or chest freezer's compressors, you're allowing heat in, making it more probable that your food will be borked before power's restored. Of course, no one ever knew when the lights would be coming back on, in the aftermath of a storm. Nor did they have any idea of how long the food being chilled could stay cold once the fridge or freezer were no longer being powered.
For his work on sorting out an estimate of how long your frozen shit will stay frozen, the gentleman in this video deserves a medal. Depending on what type of refrigerator or freezer you own, your chilly mileage may vary. But the data-driven estimate that this video provides is the best resource on the topic that I've encountered.