That 90% number seems really hard to believe.Not to me. The real number could be even higher, at least for the USA population.
Without electricity, here are just a few things that come to mind:
1) no water to your house. Have fun drinking out of the nearest river. There won't be a hospital you can go to when you get cholera or dysentery or heavy metal poisoning.
2) have fun using a chain saw (you do have one?) to chop down nearby trees so you can cook your food. How many cans of spam do you have right now in your pantry?
3) One tank of gasoline. The one in your car. That's it. Gas stations can't pump out of their tanks. It wouldn't be hard to get the 10,000 or 20,000 gallons out of their storage tanks by portable pumps. It's a day or two worth of gas. Many stations get daily gas deliveries, they don't have much inventory around.
4) No electricity means refineries can't operate. No diesel for trucks to deliver the gasoline that refineries can't produce. How many refineries can operate for long, if at all, without external electricity?
5) A lot of oil and gasoline moves by pipelines. These need significant energy for pumping along the way. No electricity, no pipelines.
6) No food being shipped from farms. No energy for factories to process it, e.g. refine grain and slaughter cattle.
7) How long can diesel-electric trains run with the diesel they have on hand? A few weeks? Of course, subways and overhead-electric trains would be totally non-functional.
8) Have fun in urban areas like NYC. I can't imagine what a place like that would be after a week or two without water or power and without working sewage.
I'll just stop right there. I could probably come up with another 100 things without really trying.
On the bright side, who will be OK:
1) A lot of the third world. People whose daily existence depends on subsistence farming can survive for years. Many of them don't have electricity right now.
2) Some farmers in the USA. They probably have hand-pumped wells (a relic from bygone days) and plenty of food on hand.
3) Some groups like the Mormons. They (at least on paper) prepare for those kinds of things.
4) People like the Amish. They would probably do OK.
5) Survivalists in general. They have stocked up on food, and they have guns. They will be happy to shoot you dead if you come to them to take their food. And if they run out of food they will be happy to go looking for you and shoot you dead and take your food.
Just 150 years ago none of this would have been an issue. We didn't have electricity and we had much lower population density.
But now, if we lose electricity nationwide for an extended period of time, we are way beyond fucked. We're mostly dead.