I have the professional experience to share that a multi-regional-scale power utility cyberattack in the USA would be extremely difficult to pull off, even more difficult to pull off discretely, and even more difficult still to keep power off for an extended period.

It would require buy in from lots of American engineers all in very different areas all of whom would not benefit from losing power in this manner.

EMP attacks are more likely to be successful at knocking out the broader grid than cyberattacks. Govt induced 'solar flares'. 

Everyone should take comfort from this netflix film revealing just how little these jackasses understand about how the grid details work.