Power back up in condo

I live in a condo and want to get a freezer to store food for an emergency. I can’t put anything outside because of HOA rules. I want power back up for my freezer in case power is out for a week. I realize food may stay frozen or at least safe for a couple of days but much longer and I would be concerned. Specifically what do you recommend as back up. I suppose it needs to be battery back up but specifically what? Thank you