“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
There are A LOT of BIG countries with big electric grids in the world today. Which countries GRIDS get hit the worst depends on which side of the Earth is facing the ‘hit’. Could the West (US, Brazil) or Europe or the East (China, India).
It is a little more complex than that. You cannot only consider how big is a country or how big is its grid.
If the Europe would be hit by a solar storm, assuming that not all of it was hit we can recover the grid in about a month and the blackout would not be longer than maybe a week.
But a solar storm would destroy also everything else, so how big is the grid is really irrelevant when you basically have every other piece (excluded the few hardened enough) destroyed.