If you're interested in the modern risks of coronal mass injections, there's a report called "Solar Storm Risk to The North American Electrical Grid" [0] (2008) that summarizes the topic well. From page 4: "The total U.S. population at risk of extended power outage from a Carrington-level storm is between 20-40 million, with durations of 16 days to 1-2 years."
[0] http://www.lloyds.com/~/media/lloyds/reports/emerging%20risk...