You seriously underestimate your fellow Americans. Many of us have lived without power under brutal conditions for extended periods, and as a result are well prepared to do so indefinitely. Many of us have served in the military spending weeks, months or more deployed in the field under brutal conditions as well with no access to heat, power, or even a decent meal. It is not that hard, and it doesn't take long to simply adapt and be comfortable with it.
You have to eat.
The food has to be available.
There has to be a way to distribute it.
Water has to be available.
Sewage has to be taken care of.
Without electricity or communications, those things become very iffy for very large percentages of the population.
I do not think it will happen. Parts of the grid would be brought up quickly.
The number 90% has a very unlikely premise attacked to it.
Lots of people are pretty tough.
My brother and I have played around with this premise extensively.
You pretty much need an all out nuclear war to achieve it, and then we have lots of other problems as well as the power grid and communications.
Food distribution is one of the biggest problems.
But finding a way to kill off a lot of Americans is really quite difficult. Finding a way to keep the power grid down for months, is difficult as well.
I think it very unlikely to happen.
Puerto Rico is a good example of having a *partial* destruction of the grid. They, overall, are doing pretty well.