When the power grid is down now, phones are down, the internet is down, mobile networks are down, payment systems and ATMs are down, gas stations are down, refrigeration is down, etc. etc.
Basically, 3 days later civilization is down, irreversably.

[–] ricecake@sh.itjust.works 5 points 1 day ago

We actually have extensive electrical backups for almost all of those things. The landline phone system was built in an era where people were significantly more concerned with nuclear war than we are now, so it would fare pretty well.
Home Internet would be down, but the actual data centers all have multiple backups for power.
The cell system would kick most people off, but it also has a lot of backup power built into it because it's viewed as an important part of disaster response in the modern world. Part of the reason it sheds load is to keep capacity available for emergency response.

Don't get me wrong, it would be super bad, more disruptive than a typical or even atypical power outage, and last a long time, but it wouldn't irreversibly take out civilization.