I won't go into too many details as to the potential destructiveness of a solar storm of the "Carrington Event" level. Presumably if you are here you have a clue. .
Near Miss: The Solar Superstorm of July 2012
NASA Science, 23 July 2014 (hat tip: All Outdoors)
"In my view the July 2012 storm was in all respects at least as strong as the 1859 Carrington event," says [Daniel Baker of the University of Colorado]. "The only difference is, it missed."
In February 2014, physicist Pete Riley of Predictive Science Inc. published a paper in Space Weather entitled "On the probability of occurrence of extreme space weather events." In it, he analyzed records of solar storms going back 50+ years. By extrapolating the frequency of ordinary storms to the extreme, he calculated the odds that a Carrington-class storm would hit Earth in the next ten years.
The answer: 12%.
One item I will note, that the All Outdoors posting on it (see hat tip above) seems to think that affordable protection could be had for the price of millions.
If a full blown event, which historically lasted for days, and have the potential to hit the entire globe, were to strike, "paltry millions" of protection are not going to help much. The Lloyds of London report he discusses apparently misses the fact that no only will the transformers be damaged, but that long conductors, particularly open air high tension wires that are set up like a perfect antenna, are going to melt.