The "house" PA system in most nursing and retirement homes is set up for the speaking voice, for announcements to be made. This usually means the system doesn't have much capability for sound pitch equalization, or it's set with the treble way up. Often nobody but the "maintenance guy" knows how to adjust it or even has the key to the cabinet where the amp itself is kept. If you haven't allowed for an extra 15 minutes to track down the "keeper of the sound system" and/or get permission to tamper with the settings, etc., you may have to live with settings that don't flatter the alto or soprano voice.

Also house systems usually allow for only one mic/input, so you won't be able to amplify both your voice and your instrument, if that's what you need.

One more thing I keep in my car trunk is a boom mike stand. It's a lot more comfortable to play guitar, dulcimer, etc., while singing into a mic on a boom than into one on a straight stand, and most senior facilities don't have booms.