>> The Texas grid is totally separate <<
Not necessarily a factor. The Texas transformers can be burned out by an EMP simultaneously with the non-Texas transformers.
Either an EMP or a massive solar event (ala Carrington Event) would send the US back into the 19th century. Estimates are that up to 90% of the US population would die. And our government is doing zip. Read all about it in Apocalypse Rising - a fantastic read.