I think the 90% dead would be within 3-4 months; sooner if it took place during cold winter months.
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Yep! Many people, some on FR, seem not to understand that a full-blown meltdown of the country’s power grid could, and probably would, cause murders on a massive scale, even cannibalism, as the food supply vanishes. People in hospitals on life support would die by the boatload, fuel supplies for the operation of homes, businesses, and motorized vehicles would become unobtainable. Deadly diseases would become widespread. So yeah, I think your estimates are quite realistic.
Yes, it would be horrendous. And many of the survivors would be relative short term survivors. Not many would last for longer than a few years.