It turns out there's a handy, comprehensive guide at FoodSafety.gov. It lists, item by item, what can endure being partially thawed, what can be kept, and what should be tossed.

The general rule in a power outage is that a full, properly sealed freezer will keep things nice and safe for about 48 hours. A half-full freezer is good for about 24 hours.

This doesn't mean things won't thaw slightly, but it does mean they should stay below the threshold where harmful bacteria can flourish (40 degrees Fahrenheit).

Avoid opening freezer doors during power outages. It's tempting to check on things, but each time you open the doors, you're letting the cold air out and the warm air in.