They're immensely difficult to restart once the aluminium solidifies: difficult enough that the answer is throw it out and make a new one. But there's a big enough difference between operational temperatures and solid that you can deprive them of electricity for e.g. 8 hours while you restart your other power generation infrastructure. IIRC for some aluminium smelters you can even run the reaction in reverse briefly if you need to jump start something.

(citations needed.... I'm recalling this from a story someone told me ~15 years ago about the Alcoa plant in Victoria, Australia)