Our biggest threats are:
1) A massive hacking of modern stations coupled with actual ground actions at key points.
2) A massive EMP attack using specialized weapons to affect a large area.
Either of these could result in massive blackouts that could last months, or even over a year, due to the difficulties in getting certain parts, not to mention the manpower needed to do restoration (and of course, that cost then being passed on to the consumer). And that's IF society doesn't go down the crapper after a couple weeks of no power and no real end in sight (which it would).