One in the right place (outside the atmosphere) would take out 1/3 to 1/2 the country. We no longer make industrial size transformers and these would most definitely blow from the strike. Last I read, 6 months minimum to replace them.
Three in the right places and the country, most of Canada, and some of Mexico are done...
Ultimately, nobody really knows for sure except for some obscure information from an "accident" that occurred in Russia back in the 80's (or was it the 70s). Most of that info is under lock and key in Russia.