How long before the power is turned off
Coal is used to generate just over 50% of the United States electricity. The typical supply stockpile is around 50 days. This equates to almost 150 million tons of coal on hand: with the specific usage sited as being 2-3/4 tons per day. This of course does not include whatever amount is in transit.
It should be noted that coal can be liquefied and turned into fuel. This was done by the Nazis and also later by South Africa. South Africa today gets around 30% of its liquid fuel needs from coal. So it cannot be taken for granted that coal will be untouched in certain collapse scenarios.
There is around 35 days of petroleum (presuming the fuel oil generation plants could make use of it), so that might be stretched by not using it for transportation.