Do most nursing homes have a standby power supply?

It may vary depending on the level of care the facility provides and local codes.

I worked at the power company and it is shocking how some senior facilities do not have a backup generator. Any senior facility should be like a hospital but that is not always the case. It is a very good question you are asking. I sometimes think I need one for my home as well.

I would think there are generators. But why not ask the facility.

I worked in dialysis for years and the centers were not required to have a back up generator, which I thought was absurd. I don't know if that guideline has changed.
You can seek clarification by going to the CMMS website, find Nursing Home Conditions of Coverage and read through those to see if Medicare/Medicaid certified nursing homes must have a back up generator. You will likely get your answer there.

Yes, there are generators hooked up to all the red outlets you see around the walls. Emergency lighting comes on, very dim and the facility can run like that indefinitely.

You have to ask each place. Some do, some don't. It's a critical question though.

During SuperStorm Sandy I had both my Mom and MIL in different assisted living facilities. The place where my Mom was had a back up generator and an arrangement for gas to be delivered when they used their supply. They covered all of the large windows with plywood, took in all the outside potted plants, chairs, etc. They also had food supplies delivered and it included things that could work if refrigeration went out.
AND to show the difference, my MIL's place had NO precautions in place, had no back up generator and was without electricity for 48 hours (actually not bad in view of some communities in the area!) Nursing homes may have different requirements but you can always ask.

Rwh000 -? are you in a coastal area & is your ? is about what likely happens if a mandatory evac is called? Like for a hurricane & your in the NWS "cone". If so, can you post that? I'll share my thoughts as we went through Katrina with MIL in a NH & withwhat has happened since regarding facilities emergency preparedness.

I think it would depend on the Code requirements for that area, and the age of the building.

Where my Dad lived for Independent Living and later Assisted Living, behind the building was a huge backup generator.