“The exercise was held from May 8 to 9, 2024, at the Johns Hopkins Applied Physics Laboratory (APL) in Laurel, Maryland, and at a Federal Emergency Management Agency (FEMA) site in Denver, Colorado.”
Article refers to a PDF of the report it’s based on:
https://www.jhuapl.edu/sites/default/files/2025-04/Space-Weather-TTX-Report-Summary-v3-FINAL.pdf
It’s worth noting that even though a building might have solar, the systems usually disable themselves in the event of a blackout to prevent back feeding into the grid.
That’s known as a grid tie system and my edit mentions that. The only way it’s going to help is if the grid is physically disconnected from the building as in the wire is not connected to the building at any point.
That wouldn’t be an off grid building then.