• your classic zombie apocalypse (99.9% die in the first 24 hours, and survivors stop going to work)
  • a developed country (say USA)
  • coal power stations

how long after people stopped doing stuff to keep the grid up (all work/monitoring/repair/maintenance ceases) would the power cut out (power “still on” means “my toaster still works”)

My guess is s couple of days, but I would like to hear from someone in the electricity supply industry.

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    $\begingroup$ Coal power plants need to be fed round the clock. So, if the firing is not done automatically, their power would could out after a few hours. Otherwise it'd take a few days tops. But: Since the majority of people is dead and has no more need for electricity, there is way more power produced than consumed, thus leading to an unstable and ultimately failing grid. Give or take 3h I'd estimate, until power cuts out because of this. Oh, and don't get me started on the nuclear power plants. Those are practically bombs waiting to go off. $\endgroup$ – Erik Mar 1 '19 at 7:50
  • $\begingroup$ With electricity grid do you mean the energy being supplied, the physical infrastructure, or both? $\endgroup$ – L.Dutch - Reinstate Monica Mar 1 '19 at 8:22
  • $\begingroup$ Power still on in homes. Q edited. $\endgroup$ – Bohemian Mar 1 '19 at 8:49
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    $\begingroup$ @Erik I'm not an expert on nuke plants, but I knew a guy who'd been a guard at one. AK-47s. Nuke plant staffs have a shot at surviving the first 24 hours of a zombie apocalypse. $\endgroup$ – Jedediah Mar 1 '19 at 10:37
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    $\begingroup$ I don't think this is a duplicate: the electrical grid is more than just a collection of power plants (and will fall apart much faster than any single plant will shut down). $\endgroup$ – Mark Mar 1 '19 at 20:38

Hours at most.

The electrical grid is a constant balancing act to match the amount of electricity generated with the amount that's consumed. There's a great deal of automation here, but it still requires human intervention, particularly when dealing with the unexpected.

99.9% of the population dying in a 24-hour period is certainly one of those unexpected situations. There will be an abrupt drop in power consumption, and once the ability of automated systems to compensate is exceeded, the grid voltage will rise. This will trigger various safeguards: sections of the grid will disconnect from each other and power plants will shut down.

Since you specify coal power plants, things will be especially bad. Coal can't adapt to rapid changes in demand the way gas or hydroelectric can. There's almost no hope that isolated pieces will stabilize themselves long enough to run out of coal. Instead, all you'll have left are individual buildings with their own solar or wind systems.

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  • $\begingroup$ Solar plants rely on the network to match their phase. $\endgroup$ – L.Dutch - Reinstate Monica Mar 1 '19 at 8:57
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    $\begingroup$ @L.Dutch, only if there's a grid to match to. Most people who have personal solar will have a grid-transfer switch that will let them run completely independent of the grid if there's a power outage. $\endgroup$ – Mark Mar 1 '19 at 9:06
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    $\begingroup$ We have photovoltaic cells that heat the water in our house. I was told we're not allowed to call them solar panels as they don't connect to the grid to feed energy back in. As long as you replace the batteries in the controller unit they would carry on working for years. $\endgroup$ – Smock Mar 1 '19 at 9:53
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    $\begingroup$ Why do you assume that people dying will affect power consumption? If you're dying, are you really going to make sure you close your laptop and shut off the tv? $\endgroup$ – TMN Mar 1 '19 at 16:46
  • $\begingroup$ @TMN, at any given time, there are many power consumers that turn themselves off automatically but get turned on by humans (think: microwaves, motion-activated lights, and many industrial systems). There are also many consumers that increase their consumption in response to human activity (think: an air-conditioner running more often because there are a half-dozen people heating up the room). $\endgroup$ – Mark Mar 1 '19 at 20:35

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