Good question. The answer would depend on how long the grid was out and when the outage occurred.
A three month outage over a winter in the north or a summer outage anywhere would be a disaster. No heat in the winter because forced air furnaces do not work without electricity running the fan. Fuel oil will not get delivered. So, expect a lot of deaths from freezing or heat.
A far greater threat would come from a lack of food. Urban groceries have a three day supply of food and even Walmart's logistics would not allow them to supply all their stores in a total power outage. Water and sewer plants would also have a tough time without electricity.
So, a general, widespread, extended power outage would result in millions of deaths in urban areas. I haven't given that ultimate number a lot of thought but a 50-95% death rate doesn't seem unreasonable.
If you can survive the first three months, you might be okay. As the systems came back on line there would be far fewer mouths to feed, especially in the urban areas.
I've seen this three day supply of food mentioned in several articles and have wondered about the source.
Anyone who has paid attention to to the news in times of natural disasters and/or civil unrest has watched video footage and has seen mobs strip these stores clean in a matter of an hour or two, leaving empty shells of buildings with broken glass and broken doors. A full out power outage and these stores are out for the duration. Doubtful that they would ever be restored even after a power recovery as the demographics of the areas served would have been drastically altered.
Having lived in Manhattan for several decades I can attest to the general paucity of suburban type supermarkets. Small mom and pop grocery stores are the rule. But the real weak point is that most urbanites rely on local restaurants and take out places for most of their meals. With non-functioning cold boxes they're out of action in 24 hours.
I pray that this happens. We could use a good urban-culling.
What with new laws, many private septic systems now are powered by electricity. It used to be you’d just have a pipe running down hill and have the hole cleaned out every few years but not so these days.